path: root/documentation/profile-manual/profile-manual-usage.xml

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<chapter id='profile-manual-usage'>

<title>Basic Usage (with examples) for each of the Yocto Tracing Tools</title>

<para>
    This chapter presents basic usage examples for each of the tracing
    tools.
</para>

<section id='profile-manual-perf'>
    <title>perf</title>

    <para>
        The 'perf' tool is the profiling and tracing tool that comes
        bundled with the Linux kernel.
    </para>

    <para>
        Don't let the fact that it's part of the kernel fool you into thinking
        that it's only for tracing and profiling the kernel - you can indeed
        use it to trace and profile just the kernel , but you can also use it
        to profile specific applications separately (with or without kernel
        context), and you can also use it to trace and profile the kernel
        and all applications on the system simultaneously to gain a system-wide
        view of what's going on.
    </para>

    <para>
        In many ways, it aims to be a superset of all the tracing and profiling
        tools available in Linux today, including all the other tools covered
        in this HOWTO. The past couple of years have seen perf subsume a lot
        of the functionality of those other tools, and at the same time those
        other tools have removed large portions of their previous functionality
        and replaced it with calls to the equivalent functionality now
        implemented by the perf subsystem. Extrapolation suggests that at
        some point those other tools will simply become completely redundant
        and go away; until then, we'll cover those other tools in these pages
        and in many cases show how the same things can be accomplished in
        perf and the other tools when it seems useful to do so.
    </para>

    <para>
        The coverage below details some of the most common ways you'll likely
        want to apply the tool; full documentation can be found either within
        the tool itself or in the man pages at
        <ulink url='http://linux.die.net/man/1/perf'>perf(1)</ulink>.
    </para>

    <section id='perf-setup'>
        <title>Setup</title>

        <para>
            For this section, we'll assume you've already performed the basic
            setup outlined in the General Setup section.
        </para>

        <para>
            In particular, you'll get the most mileage out of perf if you
            profile an image built with INHIBIT_PACKAGE_STRIP = "1" in your
            local.conf.
        </para>

        <para>
            perf runs on the target system for the most part. You can archive
            profile data and copy it to the host for analysis, but for the
            rest of this document we assume you've ssh'ed to the host and
            will be running the perf commands on the target.
        </para>
    </section>

    <section id='perf-basic-usage'>
        <title>Basic Usage</title>

        <para>
            The perf tool is pretty much self-documenting. To remind yourself
            of the available commands, simply type 'perf', which will show you
            basic usage along with the available perf subcommands:
            <literallayout class='monospaced'>
     root@crownbay:~# perf

     usage: perf [--version] [--help] COMMAND [ARGS]

     The most commonly used perf commands are:
       annotate        Read perf.data (created by perf record) and display annotated code
       archive         Create archive with object files with build-ids found in perf.data file
       bench           General framework for benchmark suites
       buildid-cache   Manage build-id cache.
       buildid-list    List the buildids in a perf.data file
       diff            Read two perf.data files and display the differential profile
       evlist          List the event names in a perf.data file
       inject          Filter to augment the events stream with additional information
       kmem            Tool to trace/measure kernel memory(slab) properties
       kvm             Tool to trace/measure kvm guest os
       list            List all symbolic event types
       lock            Analyze lock events
       probe           Define new dynamic tracepoints
       record          Run a command and record its profile into perf.data
       report          Read perf.data (created by perf record) and display the profile
       sched           Tool to trace/measure scheduler properties (latencies)
       script          Read perf.data (created by perf record) and display trace output
       stat            Run a command and gather performance counter statistics
       test            Runs sanity tests.
       timechart       Tool to visualize total system behavior during a workload
       top             System profiling tool.

     See 'perf help COMMAND' for more information on a specific command.
            </literallayout>
        </para>

        <section id='using-perf-to-do-basic-profiling'>
            <title>Using perf to do Basic Profiling</title>

            <para>
                As a simple test case, we'll profile the 'wget' of a fairly large
                file, which is a minimally interesting case because it has both
                file and network I/O aspects, and at least in the case of standard
                Yocto images, it's implemented as part of busybox, so the methods
                we use to analyze it can be used in a very similar way to the whole
                host of supported busybox applets in Yocto.
                <literallayout class='monospaced'>
     root@crownbay:~# rm linux-2.6.19.2.tar.bz2; \
     wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
                </literallayout>
                The quickest and easiest way to get some basic overall data about
                what's going on for a particular workload it to profile it using
                'perf stat'. 'perf stat' basically profiles using a few default
                counters and displays the summed counts at the end of the run:
                <literallayout class='monospaced'>
     root@crownbay:~# perf stat wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
     Connecting to downloads.yoctoproject.org (140.211.169.59:80)
     linux-2.6.19.2.tar.b 100% |***************************************************| 41727k  0:00:00 ETA

     Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2':

           4597.223902 task-clock                #    0.077 CPUs utilized
                 23568 context-switches          #    0.005 M/sec
                    68 CPU-migrations            #    0.015 K/sec
                   241 page-faults               #    0.052 K/sec
            3045817293 cycles                    #    0.663 GHz
       &lt;not supported&gt; stalled-cycles-frontend
       &lt;not supported&gt; stalled-cycles-backend
             858909167 instructions              #    0.28  insns per cycle
             165441165 branches                  #   35.987 M/sec
              19550329 branch-misses             #   11.82% of all branches

          59.836627620 seconds time elapsed
                </literallayout>
                Many times such a simple-minded test doesn't yield much of
                interest, but sometimes it does (see Real-world Yocto bug
                (slow loop-mounted write speed)).
            </para>

            <para>
                Also, note that 'perf stat' isn't restricted to a fixed set of
                counters - basically any event listed in the output of 'perf list'
                can be tallied by 'perf stat'. For example, suppose we wanted to
                see a summary of all the events related to kernel memory
                allocation/freeing along with cache hits and misses:
                <literallayout class='monospaced'>
     root@crownbay:~# perf stat -e kmem:* -e cache-references -e cache-misses wget http://    downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
     Connecting to downloads.yoctoproject.org (140.211.169.59:80)
     linux-2.6.19.2.tar.b 100% |***************************************************| 41727k  0:00:00 ETA

     Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2':

                  5566 kmem:kmalloc
                125517 kmem:kmem_cache_alloc
                     0 kmem:kmalloc_node
                     0 kmem:kmem_cache_alloc_node
                 34401 kmem:kfree
                 69920 kmem:kmem_cache_free
                   133 kmem:mm_page_free
                    41 kmem:mm_page_free_batched
                 11502 kmem:mm_page_alloc
                 11375 kmem:mm_page_alloc_zone_locked
                     0 kmem:mm_page_pcpu_drain
                     0 kmem:mm_page_alloc_extfrag
              66848602 cache-references
               2917740 cache-misses              #    4.365 % of all cache refs

          44.831023415 seconds time elapsed
                </literallayout>
                So 'perf stat' gives us a nice easy way to get a quick overview of
                what might be happening for a set of events, but normally we'd
                need a little more detail in order to understand what's going on
                in a way that we can act on in a useful way.
            </para>

            <para>
                To dive down into a next level of detail, we can use 'perf
                record'/'perf report' which will collect profiling data and
                present it to use using an interactive text-based UI (or
                simply as text if we specify --stdio to 'perf report').
            </para>

            <para>
                As our first attempt at profiling this workload, we'll simply
                run 'perf record', handing it the workload we want to profile
                (everything after 'perf record' and any perf options we hand
                it - here none - will be executedin a new shell). perf collects
                samples until the process exits and records them in a file named
                'perf.data' in the current working directory.
                <literallayout class='monospaced'>
     root@crownbay:~# perf record wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2

     Connecting to downloads.yoctoproject.org (140.211.169.59:80)
     linux-2.6.19.2.tar.b 100% |************************************************| 41727k  0:00:00 ETA
     [ perf record: Woken up 1 times to write data ]
     [ perf record: Captured and wrote 0.176 MB perf.data (~7700 samples) ]
            </literallayout>
            To see the results in a 'text-based UI' (tui), simply run
            'perf report', which will read the perf.data file in the current
            working directory and display the results in an interactive UI:
                <literallayout class='monospaced'>
     root@crownbay:~# perf report
                </literallayout>
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-flat-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                The above screenshot displays a 'flat' profile, one entry for
                each 'bucket' corresponding to the functions that were profiled
                during the profiling run, ordered from the most popular to the
                least (perf has options to sort in various orders and keys as
                well as display entries only above a certain threshold and so
                on - see the perf documentation for details). Note that this
                includes both userspace functions (entries containing a [.]) and
                kernel functions accounted to the process (entries containing
                a [k]). (perf has command-line modifiers that can be used to
                restrict the profiling to kernel or userspace, among others).
            </para>

            <para>
                Notice also that the above report shows an entry for 'busybox',
                which is the executable that implements 'wget' in Yocto, but that
                instead of a useful function name in that entry, it displays
                an not-so-friendly hex value instead. The steps below will show
                how to fix that problem.
            </para>

            <para>
                Before we do that, however, let's try running a different profile,
                one which shows something a little more interesting. The only
                difference between the new profile and the previous one is that
                we'll add the -g option, which will record not just the address
                of a sampled function, but the entire callchain to the sampled
                function as well:
                <literallayout class='monospaced'>
     root@crownbay:~# perf record -g wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
     Connecting to downloads.yoctoproject.org (140.211.169.59:80)
     linux-2.6.19.2.tar.b 100% |************************************************| 41727k  0:00:00 ETA
     [ perf record: Woken up 3 times to write data ]
     [ perf record: Captured and wrote 0.652 MB perf.data (~28476 samples) ]


     root@crownbay:~# perf report
                </literallayout>
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-g-copy-to-user-expanded-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                Using the callgraph view, we can actually see not only which
                functions took the most time, but we can also see a summary of
                how those functions were called and learn something about how the
                program interacts with the kernel in the process.
            </para>

            <para>
                Notice that each entry in the above screenshot now contains a '+'
                on the left-hand side. This means that we can expand the entry and
                drill down into the callchains that feed into that entry.
                Pressing 'enter' on any one of them will expand the callchain
                (you can also press 'E' to expand them all at the same time or 'C'
                to collapse them all).
            </para>

            <para>
                In the screenshot above, we've toggled the __copy_to_user_ll()
                entry and several subnodes all the way down. This lets us see
                which callchains contributed to the profiled __copy_to_user_ll()
                function which contributed 1.77% to the total profile.
            </para>

            <para>
                As a bit of background explanation for these callchains, think
                about what happens at a high level when you run wget to get a file
                out on the network. Basically what happens is that the data comes
                into the kernel via the network connection (socket) and is passed
                to the userspace program 'wget' (which is actually a part of
                busybox, but that's not important for now), which takes the buffers
                the kernel passes to it and writes it to a disk file to save it.
            </para>

            <para>
                The part of this process that we're looking at in the above call
                stacks is the part where the kernel passes the data it's read from
                the socket down to wget i.e. a copy-to-user.
            </para>

            <para>
                Notice also that here there's also a case where the a hex value
                is displayed in the callstack, here in the expanded
                sys_clock_gettime() function. Later we'll see it resolve to a
                userspace function call in busybox.
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-g-copy-from-user-expanded-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                The above screenshot shows the other half of the journey for the
                data - from the wget program's userspace buffers to disk. To get
                the buffers to disk, the wget program issues a write(2), which
                does a copy-from-user to the kernel, which then takes care via
                some circuitous path (probably also present somewhere in the
                profile data), to get it safely to disk.
            </para>

            <para>
                Now that we've seen the basic layout of the profile data and the
                basics of how to extract useful information out of it, let's get
                back to the task at hand and see if we can get some basic idea
                about where the time is spent in the program we're profiling,
                wget. Remember that wget is actually implemented as an applet
                in busybox, so while the process name is 'wget', the executable
                we're actually interested in is busybox. So let's expand the
                first entry containing busybox:
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-busybox-expanded-stripped.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                Again, before we expanded we saw that the function was labeled
                with a hex value instead of a symbol as with most of the kernel
                entries. Expanding the busybox entry doesn't make it any better.
            </para>

            <para>
                The problem is that perf can't find the symbol information for the
                busybox binary, which is actually stripped out by the Yocto build
                system.
            </para>

            <para>
                One way around that is to put the following in your local.conf
                when you build the image:
                <literallayout class='monospaced'>
     INHIBIT_PACKAGE_STRIP = "1"
                </literallayout>
                However, we already have an image with the binaries stripped,
                so what can we do to get perf to resolve the symbols? Basically
                we need to install the debuginfo for the busybox package.
            </para>

            <para>
                To generate the debug info for the packages in the image, we can
                to add dbg-pkgs to EXTRA_IMAGE_FEATURES in local.conf. For example:
                <literallayout class='monospaced'>
     EXTRA_IMAGE_FEATURES = "debug-tweaks tools-profile dbg-pkgs"
                </literallayout>
                Additionally, in order to generate the type of debuginfo that
                perf understands, we also need to add the following to local.conf:
                <literallayout class='monospaced'>
     PACKAGE_DEBUG_SPLIT_STYLE = 'debug-file-directory'
                </literallayout>
                Once we've done that, we can install the debuginfo for busybox.
                The debug packages once built can be found in
                build/tmp/deploy/rpm/* on the host system. Find the
                busybox-dbg-...rpm file and copy it to the target. For example:
                <literallayout class='monospaced'>
     [trz@empanada core2]$ scp /home/trz/yocto/crownbay-tracing-dbg/build/tmp/deploy/rpm/core2/busybox-dbg-1.20.2-r2.core2.rpm root@192.168.1.31:
     root@192.168.1.31's password:
     busybox-dbg-1.20.2-r2.core2.rpm                     100% 1826KB   1.8MB/s   00:01
                </literallayout>
                Now install the debug rpm on the target:
                <literallayout class='monospaced'>
     root@crownbay:~# rpm -i busybox-dbg-1.20.2-r2.core2.rpm
                </literallayout>
                Now that the debuginfo is installed, we see that the busybox
                entries now display their functions symbolically:
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-busybox-debuginfo.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                If we expand one of the entries and press 'enter' on a leaf node,
                we're presented with a menu of actions we can take to get more
                information related to that entry:
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-busybox-dso-zoom-menu.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                One of these actions allows us to show a view that displays a
                busybox-centric view of the profiled functions (in this case we've
                also expanded all the nodes using the 'E' key):
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-busybox-dso-zoom.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                Finally, we can see that now that the busybox debuginfo is
                installed, the previously unresolved symbol in the
                sys_clock_gettime() entry mentioned previously is now resolved,
                and shows that the sys_clock_gettime system call that was the
                source of 6.75% of the copy-to-user overhead was initiated by
                the handle_input() busybox function:
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-g-copy-to-user-expanded-debuginfo.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                At the lowest level of detail, we can dive down to the assembly
                level and see which instructions caused the most overhead in a
                function. Pressing 'enter' on the 'udhcpc_main' function, we're
                again presented with a menu:
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-busybox-annotate-menu.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                Selecting 'Annotate udhcpc_main', we get a detailed listing of
                percentages by instruction for the udhcpc_main function. From the
                display, we can see that over 50% of the time spent in this
                function is taken up by a couple tests and the move of a
                constant (1) to a register:
            </para>

            <para>
                <imagedata fileref="figures/perf-wget-busybox-annotate-udhcpc.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                As a segue into tracing, let's try another profile using a
                different counter, something other than the default 'cycles'.
            </para>

            <para>
                The tracing and profiling infrastructure in Linux has become
                unified in a way that allows us to use the same tool with a
                completely different set of counters, not just the standard
                hardware counters that traditionally tools have had to restrict
                themselves to (of course the traditional tools can also make use
                of the expanded possibilities now available to them, and in some
                cases have, as mentioned previously).
            </para>

            <para>
                We can get a list of the available events that can be used to
                profile a workload via 'perf list':
                <literallayout class='monospaced'>
     root@crownbay:~# perf list

     List of pre-defined events (to be used in -e):
      cpu-cycles OR cycles                               [Hardware event]
      stalled-cycles-frontend OR idle-cycles-frontend    [Hardware event]
      stalled-cycles-backend OR idle-cycles-backend      [Hardware event]
      instructions                                       [Hardware event]
      cache-references                                   [Hardware event]
      cache-misses                                       [Hardware event]
      branch-instructions OR branches                    [Hardware event]
      branch-misses                                      [Hardware event]
      bus-cycles                                         [Hardware event]
      ref-cycles                                         [Hardware event]

      cpu-clock                                          [Software event]
      task-clock                                         [Software event]
      page-faults OR faults                              [Software event]
      minor-faults                                       [Software event]
      major-faults                                       [Software event]
      context-switches OR cs                             [Software event]
      cpu-migrations OR migrations                       [Software event]
      alignment-faults                                   [Software event]
      emulation-faults                                   [Software event]

      L1-dcache-loads                                    [Hardware cache event]
      L1-dcache-load-misses                              [Hardware cache event]
      L1-dcache-prefetch-misses                          [Hardware cache event]
      L1-icache-loads                                    [Hardware cache event]
      L1-icache-load-misses                              [Hardware cache event]
      .
      .
      .
      rNNN                                               [Raw hardware event descriptor]
      cpu/t1=v1[,t2=v2,t3 ...]/modifier                  [Raw hardware event descriptor]
       (see 'perf list --help' on how to encode it)

      mem:&lt;addr&gt;[:access]                                [Hardware breakpoint]

      sunrpc:rpc_call_status                             [Tracepoint event]
      sunrpc:rpc_bind_status                             [Tracepoint event]
      sunrpc:rpc_connect_status                          [Tracepoint event]
      sunrpc:rpc_task_begin                              [Tracepoint event]
      skb:kfree_skb                                      [Tracepoint event]
      skb:consume_skb                                    [Tracepoint event]
      skb:skb_copy_datagram_iovec                        [Tracepoint event]
      net:net_dev_xmit                                   [Tracepoint event]
      net:net_dev_queue                                  [Tracepoint event]
      net:netif_receive_skb                              [Tracepoint event]
      net:netif_rx                                       [Tracepoint event]
      napi:napi_poll                                     [Tracepoint event]
      sock:sock_rcvqueue_full                            [Tracepoint event]
      sock:sock_exceed_buf_limit                         [Tracepoint event]
      udp:udp_fail_queue_rcv_skb                         [Tracepoint event]
      hda:hda_send_cmd                                   [Tracepoint event]
      hda:hda_get_response                               [Tracepoint event]
      hda:hda_bus_reset                                  [Tracepoint event]
      scsi:scsi_dispatch_cmd_start                       [Tracepoint event]
      scsi:scsi_dispatch_cmd_error                       [Tracepoint event]
      scsi:scsi_eh_wakeup                                [Tracepoint event]
      drm:drm_vblank_event                               [Tracepoint event]
      drm:drm_vblank_event_queued                        [Tracepoint event]
      drm:drm_vblank_event_delivered                     [Tracepoint event]
      random:mix_pool_bytes                              [Tracepoint event]
      random:mix_pool_bytes_nolock                       [Tracepoint event]
      random:credit_entropy_bits                         [Tracepoint event]
      gpio:gpio_direction                                [Tracepoint event]
      gpio:gpio_value                                    [Tracepoint event]
      block:block_rq_abort                               [Tracepoint event]
      block:block_rq_requeue                             [Tracepoint event]
      block:block_rq_issue                               [Tracepoint event]
      block:block_bio_bounce                             [Tracepoint event]
      block:block_bio_complete                           [Tracepoint event]
      block:block_bio_backmerge                          [Tracepoint event]
      .
      .
      writeback:writeback_wake_thread                    [Tracepoint event]
      writeback:writeback_wake_forker_thread             [Tracepoint event]
      writeback:writeback_bdi_register                   [Tracepoint event]
      .
      .
      writeback:writeback_single_inode_requeue           [Tracepoint event]
      writeback:writeback_single_inode                   [Tracepoint event]
      kmem:kmalloc                                       [Tracepoint event]
      kmem:kmem_cache_alloc                              [Tracepoint event]
      kmem:mm_page_alloc                                 [Tracepoint event]
      kmem:mm_page_alloc_zone_locked                     [Tracepoint event]
      kmem:mm_page_pcpu_drain                            [Tracepoint event]
      kmem:mm_page_alloc_extfrag                         [Tracepoint event]
      vmscan:mm_vmscan_kswapd_sleep                      [Tracepoint event]
      vmscan:mm_vmscan_kswapd_wake                       [Tracepoint event]
      vmscan:mm_vmscan_wakeup_kswapd                     [Tracepoint event]
      vmscan:mm_vmscan_direct_reclaim_begin              [Tracepoint event]
      .
      .
      module:module_get                                  [Tracepoint event]
      module:module_put                                  [Tracepoint event]
      module:module_request                              [Tracepoint event]
      sched:sched_kthread_stop                           [Tracepoint event]
      sched:sched_wakeup                                 [Tracepoint event]
      sched:sched_wakeup_new                             [Tracepoint event]
      sched:sched_process_fork                           [Tracepoint event]
      sched:sched_process_exec                           [Tracepoint event]
      sched:sched_stat_runtime                           [Tracepoint event]
      rcu:rcu_utilization                                [Tracepoint event]
      workqueue:workqueue_queue_work                     [Tracepoint event]
      workqueue:workqueue_execute_end                    [Tracepoint event]
      signal:signal_generate                             [Tracepoint event]
      signal:signal_deliver                              [Tracepoint event]
      timer:timer_init                                   [Tracepoint event]
      timer:timer_start                                  [Tracepoint event]
      timer:hrtimer_cancel                               [Tracepoint event]
      timer:itimer_state                                 [Tracepoint event]
      timer:itimer_expire                                [Tracepoint event]
      irq:irq_handler_entry                              [Tracepoint event]
      irq:irq_handler_exit                               [Tracepoint event]
      irq:softirq_entry                                  [Tracepoint event]
      irq:softirq_exit                                   [Tracepoint event]
      irq:softirq_raise                                  [Tracepoint event]
      printk:console                                     [Tracepoint event]
      task:task_newtask                                  [Tracepoint event]
      task:task_rename                                   [Tracepoint event]
      syscalls:sys_enter_socketcall                      [Tracepoint event]
      syscalls:sys_exit_socketcall                       [Tracepoint event]
      .
      .
      .
      syscalls:sys_enter_unshare                         [Tracepoint event]
      syscalls:sys_exit_unshare                          [Tracepoint event]
      raw_syscalls:sys_enter                             [Tracepoint event]
      raw_syscalls:sys_exit                              [Tracepoint event]
                </literallayout>
            </para>

            <note>
                Tying It Together: These are exactly the same set of events defined
                by the trace event subsystem and exposed by
                ftrace/tracecmd/kernelshark as files in
                /sys/kernel/debug/tracing/events, by SystemTap as
                kernel.trace("tracepoint_name") and (partially) accessed by LTTng.
            </note>

            <para>
                Only a subset of these would be of interest to us when looking at
                this workload, so let's choose the most likely subsystems
                (identified by the string before the colon in the Tracepoint events)
                and do a 'perf stat' run using only those wildcarded subsystems:
                <literallayout class='monospaced'>
     root@crownbay:~# perf stat -e skb:* -e net:* -e napi:* -e sched:* -e workqueue:* -e irq:* -e syscalls:* wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
     Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2':

                 23323 skb:kfree_skb
                     0 skb:consume_skb
                 49897 skb:skb_copy_datagram_iovec
                  6217 net:net_dev_xmit
                  6217 net:net_dev_queue
                  7962 net:netif_receive_skb
                     2 net:netif_rx
                  8340 napi:napi_poll
                     0 sched:sched_kthread_stop
                     0 sched:sched_kthread_stop_ret
                  3749 sched:sched_wakeup
                     0 sched:sched_wakeup_new
                     0 sched:sched_switch
                    29 sched:sched_migrate_task
                     0 sched:sched_process_free
                     1 sched:sched_process_exit
                     0 sched:sched_wait_task
                     0 sched:sched_process_wait
                     0 sched:sched_process_fork
                     1 sched:sched_process_exec
                     0 sched:sched_stat_wait
         2106519415641 sched:sched_stat_sleep
                     0 sched:sched_stat_iowait
             147453613 sched:sched_stat_blocked
           12903026955 sched:sched_stat_runtime
                     0 sched:sched_pi_setprio
                  3574 workqueue:workqueue_queue_work
                  3574 workqueue:workqueue_activate_work
                     0 workqueue:workqueue_execute_start
                     0 workqueue:workqueue_execute_end
                 16631 irq:irq_handler_entry
                 16631 irq:irq_handler_exit
                 28521 irq:softirq_entry
                 28521 irq:softirq_exit
                 28728 irq:softirq_raise
                     1 syscalls:sys_enter_sendmmsg
                     1 syscalls:sys_exit_sendmmsg
                     0 syscalls:sys_enter_recvmmsg
                     0 syscalls:sys_exit_recvmmsg
                    14 syscalls:sys_enter_socketcall
                    14 syscalls:sys_exit_socketcall
                       .
                       .
                       .
                 16965 syscalls:sys_enter_read
                 16965 syscalls:sys_exit_read
                 12854 syscalls:sys_enter_write
                 12854 syscalls:sys_exit_write
                       .
                       .
                       .

          58.029710972 seconds time elapsed
                </literallayout>
                Let's pick one of these tracepoints and tell perf to do a profile
                using it as the sampling event:
                <literallayout class='monospaced'>
     root@crownbay:~# perf record -g -e sched:sched_wakeup wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
                </literallayout>
            </para>

            <para>
                <imagedata fileref="figures/sched-wakeup-profile.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                The screenshot above shows the results of running a profile using
                sched:sched_switch tracepoint, which shows the relative costs of
                various paths to sched_wakeup (note that sched_wakeup is the
                name of the tracepoint - it's actually defined just inside
                ttwu_do_wakeup(), which accounts for the function name actually
                displayed in the profile:
                <literallayout class='monospaced'>
     /*
      * Mark the task runnable and perform wakeup-preemption.
      */
     static void
     ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
     {
          trace_sched_wakeup(p, true);
          .
          .
          .
     }
                </literallayout>
                A couple of the more interesting callchains are expanded and
                displayed above, basically some network receive paths that
                presumably end up waking up wget (busybox) when network data is
                ready.
            </para>

            <para>
                Note that because tracepoints are normally used for tracing,
                the default sampling period for tracepoints is 1 i.e. for
                tracepoints perf will sample on every event occurrence (this
                can be changed using the -c option). This is in contrast to
                hardware counters such as for example the default 'cycles'
                hardware counter used for normal profiling, where sampling
                periods are much higher (in the thousands) because profiling should
                have as low an overhead as possible and sampling on every cycle
                would be prohibitively expensive.
            </para>
        </section>

        <section id='using-perf-to-do-basic-tracing'>
            <title>Using perf to do Basic Tracing</title>

            <para>
                Profiling is a great tool for solving many problems or for
                getting a high-level view of what's going on with a workload or
                across the system. It is however by definition an approximation,
                as suggested by the most prominent word associated with it,
                'sampling'. On the one hand, it allows a representative picture of
                what's going on in the system to be cheaply taken, but on the other
                hand, that cheapness limits its utility when that data suggests a
                need to 'dive down' more deeply to discover what's really going
                on. In such cases, the only way to see what's really going on is
                to be able to look at (or summarize more intelligently) the
                individual steps that go into the higher-level behavior exposed
                by the coarse-grained profiling data.
            </para>

            <para>
                As a concrete example, we can trace all the events we think might
                be applicable to our workload:
                <literallayout class='monospaced'>
     root@crownbay:~# perf record -g -e skb:* -e net:* -e napi:* -e sched:sched_switch -e sched:sched_wakeup -e irq:*
      -e syscalls:sys_enter_read -e syscalls:sys_exit_read -e syscalls:sys_enter_write -e syscalls:sys_exit_write
      wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
                </literallayout>
                We can look at the raw trace output using 'perf script' with no
                arguments:
                <literallayout class='monospaced'>
     root@crownbay:~# perf script

           perf  1262 [000] 11624.857082: sys_exit_read: 0x0
           perf  1262 [000] 11624.857193: sched_wakeup: comm=migration/0 pid=6 prio=0 success=1 target_cpu=000
           wget  1262 [001] 11624.858021: softirq_raise: vec=1 [action=TIMER]
           wget  1262 [001] 11624.858074: softirq_entry: vec=1 [action=TIMER]
           wget  1262 [001] 11624.858081: softirq_exit: vec=1 [action=TIMER]
           wget  1262 [001] 11624.858166: sys_enter_read: fd: 0x0003, buf: 0xbf82c940, count: 0x0200
           wget  1262 [001] 11624.858177: sys_exit_read: 0x200
           wget  1262 [001] 11624.858878: kfree_skb: skbaddr=0xeb248d80 protocol=0 location=0xc15a5308
           wget  1262 [001] 11624.858945: kfree_skb: skbaddr=0xeb248000 protocol=0 location=0xc15a5308
           wget  1262 [001] 11624.859020: softirq_raise: vec=1 [action=TIMER]
           wget  1262 [001] 11624.859076: softirq_entry: vec=1 [action=TIMER]
           wget  1262 [001] 11624.859083: softirq_exit: vec=1 [action=TIMER]
           wget  1262 [001] 11624.859167: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
           wget  1262 [001] 11624.859192: sys_exit_read: 0x1d7
           wget  1262 [001] 11624.859228: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
           wget  1262 [001] 11624.859233: sys_exit_read: 0x0
           wget  1262 [001] 11624.859573: sys_enter_read: fd: 0x0003, buf: 0xbf82c580, count: 0x0200
           wget  1262 [001] 11624.859584: sys_exit_read: 0x200
           wget  1262 [001] 11624.859864: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
           wget  1262 [001] 11624.859888: sys_exit_read: 0x400
           wget  1262 [001] 11624.859935: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
           wget  1262 [001] 11624.859944: sys_exit_read: 0x400
                </literallayout>
                This gives us a detailed timestamped sequence of events that
                occurred within the workload with respect to those events.
            </para>

            <para>
                In many ways, profiling can be viewed as a subset of tracing -
                theoretically, if you have a set of trace events that's sufficient
                to capture all the important aspects of a workload, you can derive
                any of the results or views that a profiling run can.
            </para>

            <para>
                Another aspect of traditional profiling is that while powerful in
                many ways, it's limited by the granularity of the underlying data.
                Profiling tools offer various ways of sorting and presenting the
                sample data, which make it much more useful and amenable to user
                experimentation, but in the end it can't be used in an open-ended
                way to extract data that just isn't present as a consequence of
                the fact that conceptually, most of it has been thrown away.
            </para>

            <para>
                Full-blown detailed tracing data does however offer the opportunity
                to manipulate and present the information collected during a
                tracing run in an infinite variety of ways.
            </para>

            <para>
                Another way to look at it is that there are only so many ways that
                the 'primitive' counters can be used on their own to generate
                interesting output; to get anything more complicated than simple
                counts requires some amount of additional logic, which is typically
                very specific to the problem at hand. For example, if we wanted to
                make use of a 'counter' that maps to the value of the time
                difference between when a process was scheduled to run on a
                processor and the time it actually ran, we wouldn't expect such
                a counter to exist on its own, but we could derive one called say
                'wakeup_latency' and use it to extract a useful view of that metric
                from trace data. Likewise, we really can't figure out from standard
                profiling tools how much data every process on the system reads and
                writes, along with how many of those reads and writes fail
                completely. If we have sufficient trace data, however, we could
                with the right tools easily extract and present that information,
                but we'd need something other than pre-canned profiling tools to
                do that.
            </para>

            <para>
                Luckily, there is general-purpose way to handle such needs,
                called 'programming languages'. Making programming languages
                easily available to apply to such problems given the specific
                format of data is called a 'programming language binding' for
                that data and language. Perf supports two programming language
                bindings, one for Python and one for Perl.
            </para>

            <note>
                Tying It Together: Language bindings for manipulating and
                aggregating trace data are of course not a new
                idea.  One of the first projects to do this was IBM's DProbes
                dpcc compiler, an ANSI C compiler which targeted a low-level
                assembly language running on an in-kernel interpreter on the
                target system.  This is exactly analagous to what Sun's DTrace
                did, except that DTrace invented its own language for the purpose.
                Systemtap, heavily inspired by DTrace, also created its own
                one-off language, but rather than running the product on an
                in-kernel interpreter, created an elaborate compiler-based
                machinery to translate its language into kernel modules written
                in C.
            </note>

            <para>
                Now that we have the trace data in perf.data, we can use
                'perf script -g' to generate a skeleton script with handlers
                for the read/write entry/exit events we recorded:
                <literallayout class='monospaced'>
     root@crownbay:~# perf script -g python
     generated Python script: perf-script.py
                </literallayout>
                The skeleton script simply creates a python function for each
                event type in the perf.data file. The body of each function simply
                prints the event name along with its parameters. For example:
                <literallayout class='monospaced'>
     def net__netif_rx(event_name, context, common_cpu,
            common_secs, common_nsecs, common_pid, common_comm,
            skbaddr, len, name):
                    print_header(event_name, common_cpu, common_secs, common_nsecs,
                            common_pid, common_comm)

		     print "skbaddr=%u, len=%u, name=%s\n" % (skbaddr, len, name),
                </literallayout>
                We can run that script directly to print all of the events
                contained in the perf.data file:
                <literallayout class='monospaced'>
     root@crownbay:~# perf script -s perf-script.py

     in trace_begin
     syscalls__sys_exit_read     0 11624.857082795     1262 perf                  nr=3, ret=0
     sched__sched_wakeup      0 11624.857193498     1262 perf                  comm=migration/0, pid=6, prio=0,      success=1, target_cpu=0
     irq__softirq_raise       1 11624.858021635     1262 wget                  vec=TIMER
     irq__softirq_entry       1 11624.858074075     1262 wget                  vec=TIMER
     irq__softirq_exit        1 11624.858081389     1262 wget                  vec=TIMER
     syscalls__sys_enter_read     1 11624.858166434     1262 wget                  nr=3, fd=3, buf=3213019456,      count=512
     syscalls__sys_exit_read     1 11624.858177924     1262 wget                  nr=3, ret=512
     skb__kfree_skb           1 11624.858878188     1262 wget                  skbaddr=3945041280,           location=3243922184, protocol=0
     skb__kfree_skb           1 11624.858945608     1262 wget                  skbaddr=3945037824,      location=3243922184, protocol=0
     irq__softirq_raise       1 11624.859020942     1262 wget                  vec=TIMER
     irq__softirq_entry       1 11624.859076935     1262 wget                  vec=TIMER
     irq__softirq_exit        1 11624.859083469     1262 wget                  vec=TIMER
     syscalls__sys_enter_read     1 11624.859167565     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
     syscalls__sys_exit_read     1 11624.859192533     1262 wget                  nr=3, ret=471
     syscalls__sys_enter_read     1 11624.859228072     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
     syscalls__sys_exit_read     1 11624.859233707     1262 wget                  nr=3, ret=0
     syscalls__sys_enter_read     1 11624.859573008     1262 wget                  nr=3, fd=3, buf=3213018496,      count=512
     syscalls__sys_exit_read     1 11624.859584818     1262 wget                  nr=3, ret=512
     syscalls__sys_enter_read     1 11624.859864562     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
     syscalls__sys_exit_read     1 11624.859888770     1262 wget                  nr=3, ret=1024
     syscalls__sys_enter_read     1 11624.859935140     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
     syscalls__sys_exit_read     1 11624.859944032     1262 wget                  nr=3, ret=1024
                </literallayout>
                That in itself isn't very useful; after all, we can accomplish
                pretty much the same thing by simply running 'perf script'
                without arguments in the same directory as the perf.data file.
            </para>

            <para>
                We can however replace the print statements in the generated
                function bodies with whatever we want, and thereby make it
                infinitely more useful.
            </para>

            <para>
                As a simple example, let's just replace the print statements in
                the function bodies with a simple function that does nothing but
                increment a per-event count. When the program is run against a
                perf.data file, each time a particular event is encountered,
                a tally is incremented for that event. For example:
                <literallayout class='monospaced'>
     def net__netif_rx(event_name, context, common_cpu,
            common_secs, common_nsecs, common_pid, common_comm,
            skbaddr, len, name):
		          inc_counts(event_name)
                </literallayout>
                Each event handler function in the generated code is modified
                to do this. For convenience, we define a common function called
                inc_counts() that each handler calls; inc_counts simply tallies
                a count for each event using the 'counts' hash, which is a
                specialized has function that does Perl-like autovivification, a
                capability that's extremely useful for kinds of multi-level
                aggregation commonly used in processing traces (see perf's
                documentation on the Python language binding for details):
                <literallayout class='monospaced'>
     counts = autodict()

     def inc_counts(event_name):
            try:
                    counts[event_name] += 1
            except TypeError:
                    counts[event_name] = 1
                </literallayout>
                Finally, at the end of the trace processing run, we want to
                print the result of all the per-event tallies. For that, we
                use the special 'trace_end()' function:
                <literallayout class='monospaced'>
     def trace_end():
            for event_name, count in counts.iteritems():
                    print "%-40s %10s\n" % (event_name, count)
                </literallayout>
                The end result is a summary of all the events recorded in the
                trace:
                <literallayout class='monospaced'>
     skb__skb_copy_datagram_iovec                  13148
     irq__softirq_entry                             4796
     irq__irq_handler_exit                          3805
     irq__softirq_exit                              4795
     syscalls__sys_enter_write                      8990
     net__net_dev_xmit                               652
     skb__kfree_skb                                 4047
     sched__sched_wakeup                            1155
     irq__irq_handler_entry                         3804
     irq__softirq_raise                             4799
     net__net_dev_queue                              652
     syscalls__sys_enter_read                      17599
     net__netif_receive_skb                         1743
     syscalls__sys_exit_read                       17598
     net__netif_rx                                     2
     napi__napi_poll                                1877
     syscalls__sys_exit_write                       8990
                </literallayout>
                Note that this is pretty much exactly the same information we get
                from 'perf stat', which goes a little way to support the idea
                mentioned previously that given the right kind of trace data,
                higher-level profiling-type summaries can be derived from it.
            </para>

            <para>
                Documentation on using the
                <ulink url='http://linux.die.net/man/1/perf-script-python'>'perf script' python binding</ulink>.
            </para>
        </section>

        <section id='system-wide-tracing-and-profiling'>
            <title>System-Wide Tracing and Profiling</title>

            <para>
                The examples so far have focused on tracing a particular program or
                workload - in other words, every profiling run has specified the
                program to profile in the command-line e.g. 'perf record wget ...'.
            </para>

            <para>
                It's also possible, and more interesting in many cases, to run a
                system-wide profile or trace while running the workload in a
                separate shell.
            </para>

            <para>
                To do system-wide profiling or tracing, you typically use
                the -a flag to 'perf record'.
            </para>

            <para>
                To demonstrate this, open up one window and start the profile
                using the -a flag (press Ctrl-C to stop tracing):
                <literallayout class='monospaced'>
     root@crownbay:~# perf record -g -a
     ^C[ perf record: Woken up 6 times to write data ]
     [ perf record: Captured and wrote 1.400 MB perf.data (~61172 samples) ]
                </literallayout>
                In another window, run the wget test:
                <literallayout class='monospaced'>
     root@crownbay:~# wget <ulink url='http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2'>http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2</ulink>
     Connecting to downloads.yoctoproject.org (140.211.169.59:80)
     linux-2.6.19.2.tar.b 100% |*******************************| 41727k  0:00:00 ETA
                </literallayout>
                Here we see entries not only for our wget load, but for other
                processes running on the system as well:
            </para>

            <para>
                <imagedata fileref="figures/perf-systemwide.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                In the snapshot above, we can see callchains that originate in
                libc, and a callchain from Xorg that demonstrates that we're
                using a proprietary X driver in userspace (notice the presence
                of 'PVR' and some other unresolvable symbols in the expanded
                Xorg callchain).
            </para>

            <para>
                Note also that we have both kernel and userspace entries in the
                above snapshot. We can also tell perf to focus on userspace but
                providing a modifier, in this case 'u', to the 'cycles' hardware
                counter when we record a profile:
                <literallayout class='monospaced'>
     root@crownbay:~# perf record -g -a -e cycles:u
     ^C[ perf record: Woken up 2 times to write data ]
     [ perf record: Captured and wrote 0.376 MB perf.data (~16443 samples) ]
                </literallayout>
            </para>

            <para>
                <imagedata fileref="figures/perf-report-cycles-u.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                Notice in the screenshot above, we see only userspace entries ([.])
            </para>

            <para>
                Finally, we can press 'enter' on a leaf node and select the 'Zoom
                into DSO' menu item to show only entries associated with a
                specific DSO. In the screenshot below, we've zoomed into the
                'libc' DSO which shows all the entries associated with the
                libc-xxx.so DSO.
            </para>

            <para>
                <imagedata fileref="figures/perf-systemwide-libc.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <para>
                We can also use the system-wide -a switch to do system-wide
                tracing. Here we'll trace a couple of scheduler events:
                <literallayout class='monospaced'>
     root@crownbay:~# perf record -a -e sched:sched_switch -e sched:sched_wakeup
     ^C[ perf record: Woken up 38 times to write data ]
     [ perf record: Captured and wrote 9.780 MB perf.data (~427299 samples) ]
                </literallayout>
                We can look at the raw output using 'perf script' with no
                arguments:
                <literallayout class='monospaced'>
     root@crownbay:~# perf script

                perf  1383 [001]  6171.460045: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                perf  1383 [001]  6171.460066: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
         kworker/1:1    21 [001]  6171.460093: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120
             swapper     0 [000]  6171.468063: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000
             swapper     0 [000]  6171.468107: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
         kworker/0:3  1209 [000]  6171.468143: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
                perf  1383 [001]  6171.470039: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                perf  1383 [001]  6171.470058: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
         kworker/1:1    21 [001]  6171.470082: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120
                perf  1383 [001]  6171.480035: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                </literallayout>
            </para>

            <section id='perf-filtering'>
                <title>Filtering</title>

                <para>
                    Notice that there are a lot of events that don't really have
                    anything to do with what we're interested in, namely events
                    that schedule 'perf' itself in and out or that wake perf up.
                    We can get rid of those by using the '--filter' option -
                    for each event we specify using -e, we can add a --filter
                    after that to filter out trace events that contain fields
                    with specific values:
                    <literallayout class='monospaced'>
     root@crownbay:~# perf record -a -e sched:sched_switch --filter 'next_comm != perf &amp;&amp; prev_comm != perf' -e sched:sched_wakeup --filter 'comm != perf'
     ^C[ perf record: Woken up 38 times to write data ]
     [ perf record: Captured and wrote 9.688 MB perf.data (~423279 samples) ]


     root@crownbay:~# perf script

             swapper     0 [000]  7932.162180: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
         kworker/0:3  1209 [000]  7932.162236: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
                perf  1407 [001]  7932.170048: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                perf  1407 [001]  7932.180044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                perf  1407 [001]  7932.190038: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                perf  1407 [001]  7932.200044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                perf  1407 [001]  7932.210044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
                perf  1407 [001]  7932.220044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
             swapper     0 [001]  7932.230111: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
             swapper     0 [001]  7932.230146: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
         kworker/1:1    21 [001]  7932.230205: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=swapper/1 next_pid=0 next_prio=120
             swapper     0 [000]  7932.326109: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000
             swapper     0 [000]  7932.326171: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
         kworker/0:3  1209 [000]  7932.326214: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
                    </literallayout>
                    In this case, we've filtered out all events that have 'perf'
                    in their 'comm' or 'comm_prev' or 'comm_next' fields. Notice
                    that there are still events recorded for perf, but notice
                    that those events don't have values of 'perf' for the filtered
                    fields. To completely filter out anything from perf will
                    require a bit more work, but for the purpose of demonstrating
                    how to use filters, it's close enough.
                </para>

                <note>
                    Tying It Together: These are exactly the same set of event
                    filters defined by the trace event subsystem. See the
                    ftrace/tracecmd/kernelshark section for more discussion about
                    these event filters.
                </note>

                <note>
                    Tying It Together: These event filters are implemented by a
                    special-purpose pseudo-interpreter in the kernel and are an
                    integral and indispensable part of the perf design as it
                    relates to tracing.  kernel-based event filters provide a
                    mechanism to precisely throttle the event stream that appears
                    in user space, where it makes sense to provide bindings to real
                    programming languages for postprocessing the event stream.
                    This architecture allows for the intelligent and flexible
                    partitioning of processing between the kernel and user space.
                    Contrast this with other tools such as SystemTap, which does
                    all of its processing in the kernel and as such requires a
                    special project-defined language in order to accommodate that
                    design, or LTTng, where everything is sent to userspace and
                    as such requires a super-efficient kernel-to-userspace
                    transport mechanism in order to function properly.  While
                    perf certainly can benefit from for instance advances in
                    the design of the transport, it doesn't fundamentally depend
                    on them.  Basically, if you find that your perf tracing
                    application is causing buffer I/O overruns, it probably
                    means that you aren't taking enough advantage of the
                    kernel filtering engine.
                </note>
            </section>
        </section>

        <section id='using-dynamic-tracepoints'>
            <title>Using Dynamic Tracepoints</title>

            <para>
                perf isn't restricted to the fixed set of static tracepoints
                listed by 'perf list'. Users can also add their own 'dynamic'
                tracepoints anywhere in the kernel. For instance, suppose we
                want to define our own tracepoint on do_fork(). We can do that
                using the 'perf probe' perf subcommand:
                <literallayout class='monospaced'>
     root@crownbay:~# perf probe do_fork
     Added new event:
       probe:do_fork        (on do_fork)

     You can now use it in all perf tools, such as:

	     perf record -e probe:do_fork -aR sleep 1
                </literallayout>
                Adding a new tracepoint via 'perf probe' results in an event
                with all the expected files and format in
                /sys/kernel/debug/tracing/events, just the same as for static
                tracepoints (as discussed in more detail in the trace events
                subsystem section:
                <literallayout class='monospaced'>
     root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# ls -al
     drwxr-xr-x    2 root     root             0 Oct 28 11:42 .
     drwxr-xr-x    3 root     root             0 Oct 28 11:42 ..
     -rw-r--r--    1 root     root             0 Oct 28 11:42 enable
     -rw-r--r--    1 root     root             0 Oct 28 11:42 filter
     -r--r--r--    1 root     root             0 Oct 28 11:42 format
     -r--r--r--    1 root     root             0 Oct 28 11:42 id

     root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# cat format
     name: do_fork
     ID: 944
     format:
	     field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	     field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	     field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	     field:int common_pid;	offset:4;	size:4;	signed:1;
	     field:int common_padding;	offset:8;	size:4;	signed:1;

	     field:unsigned long __probe_ip;	offset:12;	size:4;	signed:0;

     print fmt: "(%lx)", REC->__probe_ip
                </literallayout>
                We can list all dynamic tracepoints currently in existence:
                <literallayout class='monospaced'>
     root@crownbay:~# perf probe -l
      probe:do_fork        (on do_fork)
      probe:schedule       (on schedule)
                </literallayout>
                Let's record system-wide ('sleep 30' is a trick for recording
                system-wide but basically do nothing and then wake up after
                30 seconds):
                <literallayout class='monospaced'>
     root@crownbay:~# perf record -g -a -e probe:do_fork sleep 30
     [ perf record: Woken up 1 times to write data ]
     [ perf record: Captured and wrote 0.087 MB perf.data (~3812 samples) ]
                </literallayout>
                Using 'perf script' we can see each do_fork event that fired:
                <literallayout class='monospaced'>
     root@crownbay:~# perf script

     # ========
     # captured on: Sun Oct 28 11:55:18 2012
     # hostname : crownbay
     # os release : 3.4.11-yocto-standard
     # perf version : 3.4.11
     # arch : i686
     # nrcpus online : 2
     # nrcpus avail : 2
     # cpudesc : Intel(R) Atom(TM) CPU E660 @ 1.30GHz
     # cpuid : GenuineIntel,6,38,1
     # total memory : 1017184 kB
     # cmdline : /usr/bin/perf record -g -a -e probe:do_fork sleep 30
     # event : name = probe:do_fork, type = 2, config = 0x3b0, config1 = 0x0, config2 = 0x0, excl_usr = 0, excl_kern
      = 0, id = { 5, 6 }
     # HEADER_CPU_TOPOLOGY info available, use -I to display
     # ========
     #
      matchbox-deskto  1197 [001] 34211.378318: do_fork: (c1028460)
      matchbox-deskto  1295 [001] 34211.380388: do_fork: (c1028460)
              pcmanfm  1296 [000] 34211.632350: do_fork: (c1028460)
              pcmanfm  1296 [000] 34211.639917: do_fork: (c1028460)
      matchbox-deskto  1197 [001] 34217.541603: do_fork: (c1028460)
      matchbox-deskto  1299 [001] 34217.543584: do_fork: (c1028460)
               gthumb  1300 [001] 34217.697451: do_fork: (c1028460)
               gthumb  1300 [001] 34219.085734: do_fork: (c1028460)
               gthumb  1300 [000] 34219.121351: do_fork: (c1028460)
               gthumb  1300 [001] 34219.264551: do_fork: (c1028460)
              pcmanfm  1296 [000] 34219.590380: do_fork: (c1028460)
      matchbox-deskto  1197 [001] 34224.955965: do_fork: (c1028460)
      matchbox-deskto  1306 [001] 34224.957972: do_fork: (c1028460)
      matchbox-termin  1307 [000] 34225.038214: do_fork: (c1028460)
      matchbox-termin  1307 [001] 34225.044218: do_fork: (c1028460)
      matchbox-termin  1307 [000] 34225.046442: do_fork: (c1028460)
      matchbox-deskto  1197 [001] 34237.112138: do_fork: (c1028460)
      matchbox-deskto  1311 [001] 34237.114106: do_fork: (c1028460)
                 gaku  1312 [000] 34237.202388: do_fork: (c1028460)
                </literallayout>
                And using 'perf report' on the same file, we can see the
                callgraphs from starting a few programs during those 30 seconds:
            </para>

            <para>
                <imagedata fileref="figures/perf-probe-do_fork-profile.png" width="6in" depth="7in" align="center" scalefit="1" />
            </para>

            <note>
                Tying It Together: The trace events subsystem accomodate static
                and dynamic tracepoints in exactly the same way - there's no
                difference as far as the infrastructure is concerned.  See the
                ftrace section for more details on the trace event subsystem.
            </note>

            <note>
                Tying It Together: Dynamic tracepoints are implemented under the
                covers by kprobes and uprobes.  kprobes and uprobes are also used
                by and in fact are the main focus of SystemTap.
            </note>
        </section>
    </section>

    <section id='perf-documentation'>
        <title>Documentation</title>

        <para>
            Online versions of the man pages for the commands discussed in this
            section can be found here:
            <itemizedlist>
                <listitem><para>The <ulink url='http://linux.die.net/man/1/perf-stat'>'perf stat' manpage</ulink>.
                    </para></listitem>
                <listitem><para>The <ulink url='http://linux.die.net/man/1/perf-record'>'perf record' manpage</ulink>.
                    </para></listitem>
                <listitem><para>The <ulink url='http://linux.die.net/man/1/perf-report'>'perf report' manpage</ulink>.
                    </para></listitem>
                <listitem><para>The <ulink url='http://linux.die.net/man/1/perf-probe'>'perf probe' manpage</ulink>.
                    </para></listitem>
                <listitem><para>The <ulink url='http://linux.die.net/man/1/perf-script'>'perf script' manpage</ulink>.
                    </para></listitem>
                <listitem><para>Documentation on using the
                    <ulink url='http://linux.die.net/man/1/perf-script-python'>'perf script' python binding</ulink>.
                    </para></listitem>
                <listitem><para>The top-level
                    <ulink url='http://linux.die.net/man/1/perf'>perf(1) manpage</ulink>.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            Normally, you should be able to invoke the man pages via perf
            itself e.g. 'perf help' or 'perf help record'.
        </para>

        <para>
            However, by default Yocto doesn't install man pages, but perf
            invokes the man pages for most help functionality. This is a bug
            and is being addressed by a Yocto bug:
            <ulink url='https://bugzilla.yoctoproject.org/show_bug.cgi?id=3388'>Bug 3388 - perf: enable man pages for basic 'help' functionality</ulink>.
        </para>

        <para>
            The man pages in text form, along with some other files, such as
            a set of examples, can be found in the 'perf' directory of the
            kernel tree:
            <literallayout class='monospaced'>
     tools/perf/Documentation
            </literallayout>
            There's also a nice perf tutorial on the perf wiki that goes
            into more detail than we do here in certain areas:
            <ulink url='https://perf.wiki.kernel.org/index.php/Tutorial'>Perf Tutorial</ulink>
        </para>
    </section>
</section>

<section id='profile-manual-ftrace'>
    <title>ftrace</title>

    <para>
        'ftrace' literally refers to the 'ftrace function tracer' but in
        reality this encompasses a number of related tracers along with
        the infrastructure that they all make use of.
    </para>

    <section id='ftrace-setup'>
        <title>Setup</title>

        <para>
            For this section, we'll assume you've already performed the basic
            setup outlined in the General Setup section.
        </para>

        <para>
            ftrace, trace-cmd, and kernelshark run on the target system,
            and are ready to go out-of-the-box - no additional setup is
            necessary. For the rest of this section we assume you've ssh'ed
            to the host and will be running ftrace on the target. kernelshark
            is a GUI application and if you use the '-X' option to ssh you
            can have the kernelshark GUI run on the target but display
            remotely on the host if you want.
        </para>
    </section>

    <section id='basic-ftrace-usage'>
        <title>Basic ftrace usage</title>

        <para>
            'ftrace' essentially refers to everything included in
            the /tracing directory of the mounted debugfs filesystem
            (Yocto follows the standard convention and mounts it
            at /sys/kernel/debug). Here's a listing of all the files
            found in /sys/kernel/debug/tracing on a Yocto system.:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# ls
     README                      kprobe_events               trace
     available_events            kprobe_profile              trace_clock
     available_filter_functions  options                     trace_marker
     available_tracers           per_cpu                     trace_options
     buffer_size_kb              printk_formats              trace_pipe
     buffer_total_size_kb        saved_cmdlines              tracing_cpumask
     current_tracer              set_event                   tracing_enabled
     dyn_ftrace_total_info       set_ftrace_filter           tracing_on
     enabled_functions           set_ftrace_notrace          tracing_thresh
     events                      set_ftrace_pid
     free_buffer                 set_graph_function
            </literallayout>
            The files listed above are used for various purposes -
            some relate directly to the tracers themselves, others are
            used to set tracing options, and yet others actually contain
            the tracing output when a tracer is in effect. Some of the
            functions can be guessed from their names, others need
            explanation; in any case, we'll cover some of the files we
            see here below but for an explanation of the others, please
            see the ftrace documentation.
        </para>

        <para>
            We'll start by looking at some of the available built-in
            tracers.
        </para>

        <para>
            cat'ing the 'available_tracers' file lists the set of
            available tracers:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# cat available_tracers
     blk function_graph function nop
            </literallayout>
            The 'current_tracer' file contains the tracer currently in
            effect:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer
     nop
            </literallayout>
            The above listing of current_tracer shows that
            the 'nop' tracer is in effect, which is just another
            way of saying that there's actually no tracer
            currently in effect.
        </para>

        <para>
            echo'ing one of the available_tracers into current_tracer
            makes the specified tracer the current tracer:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# echo function > current_tracer
     root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer
     function
            </literallayout>
            The above sets the current tracer to be the
            'function tracer'. This tracer traces every function
            call in the kernel and makes it available as the
            contents of the 'trace' file. Reading the 'trace' file
            lists the currently buffered function calls that have been
            traced by the function tracer:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# cat trace | less

     # tracer: function
     #
     # entries-in-buffer/entries-written: 310629/766471   #P:8
     #
     #                              _-----=&gt; irqs-off
     #                             / _----=&gt; need-resched
     #                            | / _---=&gt; hardirq/softirq
     #                            || / _--=&gt; preempt-depth
     #                            ||| /     delay
     #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
     #              | |       |   ||||       |         |
              &lt;idle&gt;-0     [004] d..1   470.867169: ktime_get_real &lt;-intel_idle
              &lt;idle&gt;-0     [004] d..1   470.867170: getnstimeofday &lt;-ktime_get_real
              &lt;idle&gt;-0     [004] d..1   470.867171: ns_to_timeval &lt;-intel_idle
              &lt;idle&gt;-0     [004] d..1   470.867171: ns_to_timespec &lt;-ns_to_timeval
              &lt;idle&gt;-0     [004] d..1   470.867172: smp_apic_timer_interrupt &lt;-apic_timer_interrupt
              &lt;idle&gt;-0     [004] d..1   470.867172: native_apic_mem_write &lt;-smp_apic_timer_interrupt
              &lt;idle&gt;-0     [004] d..1   470.867172: irq_enter &lt;-smp_apic_timer_interrupt
              &lt;idle&gt;-0     [004] d..1   470.867172: rcu_irq_enter &lt;-irq_enter
              &lt;idle&gt;-0     [004] d..1   470.867173: rcu_idle_exit_common.isra.33 &lt;-rcu_irq_enter
              &lt;idle&gt;-0     [004] d..1   470.867173: local_bh_disable &lt;-irq_enter
              &lt;idle&gt;-0     [004] d..1   470.867173: add_preempt_count &lt;-local_bh_disable
              &lt;idle&gt;-0     [004] d.s1   470.867174: tick_check_idle &lt;-irq_enter
              &lt;idle&gt;-0     [004] d.s1   470.867174: tick_check_oneshot_broadcast &lt;-tick_check_idle
              &lt;idle&gt;-0     [004] d.s1   470.867174: ktime_get &lt;-tick_check_idle
              &lt;idle&gt;-0     [004] d.s1   470.867174: tick_nohz_stop_idle &lt;-tick_check_idle
              &lt;idle&gt;-0     [004] d.s1   470.867175: update_ts_time_stats &lt;-tick_nohz_stop_idle
              &lt;idle&gt;-0     [004] d.s1   470.867175: nr_iowait_cpu &lt;-update_ts_time_stats
              &lt;idle&gt;-0     [004] d.s1   470.867175: tick_do_update_jiffies64 &lt;-tick_check_idle
              &lt;idle&gt;-0     [004] d.s1   470.867175: _raw_spin_lock &lt;-tick_do_update_jiffies64
              &lt;idle&gt;-0     [004] d.s1   470.867176: add_preempt_count &lt;-_raw_spin_lock
              &lt;idle&gt;-0     [004] d.s2   470.867176: do_timer &lt;-tick_do_update_jiffies64
              &lt;idle&gt;-0     [004] d.s2   470.867176: _raw_spin_lock &lt;-do_timer
              &lt;idle&gt;-0     [004] d.s2   470.867176: add_preempt_count &lt;-_raw_spin_lock
              &lt;idle&gt;-0     [004] d.s3   470.867177: ntp_tick_length &lt;-do_timer
              &lt;idle&gt;-0     [004] d.s3   470.867177: _raw_spin_lock_irqsave &lt;-ntp_tick_length
              .
              .
              .
            </literallayout>
            Each line in the trace above shows what was happening in
            the kernel on a given cpu, to the level of detail of
            function calls. Each entry shows the function called,
            followed by its caller (after the arrow).
        </para>

        <para>
            The function tracer gives you an extremely detailed idea
            of what the kernel was doing at the point in time the trace
            was taken, and is a great way to learn about how the kernel
            code works in a dynamic sense.
        </para>

        <note>
            Tying It Together: The ftrace function tracer is also
            available from within perf, as the ftrace:function tracepoint.
        </note>

        <para>
            It is a little more difficult to follow the call chains than
            it needs to be - luckily there's a variant of the function
            tracer that displays the callchains explicitly, called the
            'function_graph' tracer:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# echo function_graph &gt; current_tracer
     root@sugarbay:/sys/kernel/debug/tracing# cat trace | less

      tracer: function_graph

      CPU  DURATION                  FUNCTION CALLS
      |     |   |                     |   |   |   |
     7)   0.046 us    |      pick_next_task_fair();
     7)   0.043 us    |      pick_next_task_stop();
     7)   0.042 us    |      pick_next_task_rt();
     7)   0.032 us    |      pick_next_task_fair();
     7)   0.030 us    |      pick_next_task_idle();
     7)               |      _raw_spin_unlock_irq() {
     7)   0.033 us    |        sub_preempt_count();
     7)   0.258 us    |      }
     7)   0.032 us    |      sub_preempt_count();
     7) + 13.341 us   |    } /* __schedule */
     7)   0.095 us    |  } /* sub_preempt_count */
     7)               |  schedule() {
     7)               |    __schedule() {
     7)   0.060 us    |      add_preempt_count();
     7)   0.044 us    |      rcu_note_context_switch();
     7)               |      _raw_spin_lock_irq() {
     7)   0.033 us    |        add_preempt_count();
     7)   0.247 us    |      }
     7)               |      idle_balance() {
     7)               |        _raw_spin_unlock() {
     7)   0.031 us    |          sub_preempt_count();
     7)   0.246 us    |        }
     7)               |        update_shares() {
     7)   0.030 us    |          __rcu_read_lock();
     7)   0.029 us    |          __rcu_read_unlock();
     7)   0.484 us    |        }
     7)   0.030 us    |        __rcu_read_lock();
     7)               |        load_balance() {
     7)               |          find_busiest_group() {
     7)   0.031 us    |            idle_cpu();
     7)   0.029 us    |            idle_cpu();
     7)   0.035 us    |            idle_cpu();
     7)   0.906 us    |          }
     7)   1.141 us    |        }
     7)   0.022 us    |        msecs_to_jiffies();
     7)               |        load_balance() {
     7)               |          find_busiest_group() {
     7)   0.031 us    |            idle_cpu();
     .
     .
     .
     4)   0.062 us    |        msecs_to_jiffies();
     4)   0.062 us    |        __rcu_read_unlock();
     4)               |        _raw_spin_lock() {
     4)   0.073 us    |          add_preempt_count();
     4)   0.562 us    |        }
     4) + 17.452 us   |      }
     4)   0.108 us    |      put_prev_task_fair();
     4)   0.102 us    |      pick_next_task_fair();
     4)   0.084 us    |      pick_next_task_stop();
     4)   0.075 us    |      pick_next_task_rt();
     4)   0.062 us    |      pick_next_task_fair();
     4)   0.066 us    |      pick_next_task_idle();
     ------------------------------------------
     4)   kworker-74   =&gt;    &lt;idle&gt;-0
     ------------------------------------------

     4)               |      finish_task_switch() {
     4)               |        _raw_spin_unlock_irq() {
     4)   0.100 us    |          sub_preempt_count();
     4)   0.582 us    |        }
     4)   1.105 us    |      }
     4)   0.088 us    |      sub_preempt_count();
     4) ! 100.066 us  |    }
     .
     .
     .
     3)               |  sys_ioctl() {
     3)   0.083 us    |    fget_light();
     3)               |    security_file_ioctl() {
     3)   0.066 us    |      cap_file_ioctl();
     3)   0.562 us    |    }
     3)               |    do_vfs_ioctl() {
     3)               |      drm_ioctl() {
     3)   0.075 us    |        drm_ut_debug_printk();
     3)               |        i915_gem_pwrite_ioctl() {
     3)               |          i915_mutex_lock_interruptible() {
     3)   0.070 us    |            mutex_lock_interruptible();
     3)   0.570 us    |          }
     3)               |          drm_gem_object_lookup() {
     3)               |            _raw_spin_lock() {
     3)   0.080 us    |              add_preempt_count();
     3)   0.620 us    |            }
     3)               |            _raw_spin_unlock() {
     3)   0.085 us    |              sub_preempt_count();
     3)   0.562 us    |            }
     3)   2.149 us    |          }
     3)   0.133 us    |          i915_gem_object_pin();
     3)               |          i915_gem_object_set_to_gtt_domain() {
     3)   0.065 us    |            i915_gem_object_flush_gpu_write_domain();
     3)   0.065 us    |            i915_gem_object_wait_rendering();
     3)   0.062 us    |            i915_gem_object_flush_cpu_write_domain();
     3)   1.612 us    |          }
     3)               |          i915_gem_object_put_fence() {
     3)   0.097 us    |            i915_gem_object_flush_fence.constprop.36();
     3)   0.645 us    |          }
     3)   0.070 us    |          add_preempt_count();
     3)   0.070 us    |          sub_preempt_count();
     3)   0.073 us    |          i915_gem_object_unpin();
     3)   0.068 us    |          mutex_unlock();
     3)   9.924 us    |        }
     3) + 11.236 us   |      }
     3) + 11.770 us   |    }
     3) + 13.784 us   |  }
     3)               |  sys_ioctl() {
            </literallayout>
            As you can see, the function_graph display is much easier to
            follow. Also note that in addition to the function calls and
            associated braces, other events such as scheduler events
            are displayed in context. In fact, you can freely include
            any tracepoint available in the trace events subsystem described
            in the next section by simply enabling those events, and they'll
            appear in context in the function graph display. Quite a
            powerful tool for understanding kernel dynamics.
        </para>

        <para>
            Also notice that there are various annotations on the left
            hand side of the display. For example if the total time it
            took for a given function to execute is above a certain
            threshold, and exclamation point or plus sign appears on the
            left hand side. Please see the ftrace documentation for
            details on all these fields.
        </para>
    </section>

    <section id='the-trace-events-subsystem'>
        <title>The 'trace events' Subsystem</title>

        <para>
            One especially important directory contained within
            the /sys/kernel/debug/tracing directory is the 'events'
            subdirectory, which contains representations of every
            tracepoint in the system. Listing out the contents of
            the 'events' subdirectory, we see mainly another set of
            subdirectories:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# cd events
     root@sugarbay:/sys/kernel/debug/tracing/events# ls -al
     drwxr-xr-x   38 root     root             0 Nov 14 23:19 .
     drwxr-xr-x    5 root     root             0 Nov 14 23:19 ..
     drwxr-xr-x   19 root     root             0 Nov 14 23:19 block
     drwxr-xr-x   32 root     root             0 Nov 14 23:19 btrfs
     drwxr-xr-x    5 root     root             0 Nov 14 23:19 drm
     -rw-r--r--    1 root     root             0 Nov 14 23:19 enable
     drwxr-xr-x   40 root     root             0 Nov 14 23:19 ext3
     drwxr-xr-x   79 root     root             0 Nov 14 23:19 ext4
     drwxr-xr-x   14 root     root             0 Nov 14 23:19 ftrace
     drwxr-xr-x    8 root     root             0 Nov 14 23:19 hda
     -r--r--r--    1 root     root             0 Nov 14 23:19 header_event
     -r--r--r--    1 root     root             0 Nov 14 23:19 header_page
     drwxr-xr-x   25 root     root             0 Nov 14 23:19 i915
     drwxr-xr-x    7 root     root             0 Nov 14 23:19 irq
     drwxr-xr-x   12 root     root             0 Nov 14 23:19 jbd
     drwxr-xr-x   14 root     root             0 Nov 14 23:19 jbd2
     drwxr-xr-x   14 root     root             0 Nov 14 23:19 kmem
     drwxr-xr-x    7 root     root             0 Nov 14 23:19 module
     drwxr-xr-x    3 root     root             0 Nov 14 23:19 napi
     drwxr-xr-x    6 root     root             0 Nov 14 23:19 net
     drwxr-xr-x    3 root     root             0 Nov 14 23:19 oom
     drwxr-xr-x   12 root     root             0 Nov 14 23:19 power
     drwxr-xr-x    3 root     root             0 Nov 14 23:19 printk
     drwxr-xr-x    8 root     root             0 Nov 14 23:19 random
     drwxr-xr-x    4 root     root             0 Nov 14 23:19 raw_syscalls
     drwxr-xr-x    3 root     root             0 Nov 14 23:19 rcu
     drwxr-xr-x    6 root     root             0 Nov 14 23:19 rpm
     drwxr-xr-x   20 root     root             0 Nov 14 23:19 sched
     drwxr-xr-x    7 root     root             0 Nov 14 23:19 scsi
     drwxr-xr-x    4 root     root             0 Nov 14 23:19 signal
     drwxr-xr-x    5 root     root             0 Nov 14 23:19 skb
     drwxr-xr-x    4 root     root             0 Nov 14 23:19 sock
     drwxr-xr-x   10 root     root             0 Nov 14 23:19 sunrpc
     drwxr-xr-x  538 root     root             0 Nov 14 23:19 syscalls
     drwxr-xr-x    4 root     root             0 Nov 14 23:19 task
     drwxr-xr-x   14 root     root             0 Nov 14 23:19 timer
     drwxr-xr-x    3 root     root             0 Nov 14 23:19 udp
     drwxr-xr-x   21 root     root             0 Nov 14 23:19 vmscan
     drwxr-xr-x    3 root     root             0 Nov 14 23:19 vsyscall
     drwxr-xr-x    6 root     root             0 Nov 14 23:19 workqueue
     drwxr-xr-x   26 root     root             0 Nov 14 23:19 writeback
            </literallayout>
            Each one of these subdirectories corresponds to a
            'subsystem' and contains yet again more subdirectories,
            each one of those finally corresponding to a tracepoint.
            For example, here are the contents of the 'kmem' subsystem:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing/events# cd kmem
     root@sugarbay:/sys/kernel/debug/tracing/events/kmem# ls -al
     drwxr-xr-x   14 root     root             0 Nov 14 23:19 .
     drwxr-xr-x   38 root     root             0 Nov 14 23:19 ..
     -rw-r--r--    1 root     root             0 Nov 14 23:19 enable
     -rw-r--r--    1 root     root             0 Nov 14 23:19 filter
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 kfree
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmalloc
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmalloc_node
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmem_cache_alloc
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmem_cache_alloc_node
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmem_cache_free
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_alloc
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_alloc_extfrag
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_alloc_zone_locked
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_free
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_free_batched
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_pcpu_drain
            </literallayout>
            Let's see what's inside the subdirectory for a specific
            tracepoint, in this case the one for kmalloc:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing/events/kmem# cd kmalloc
     root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# ls -al
     drwxr-xr-x    2 root     root             0 Nov 14 23:19 .
     drwxr-xr-x   14 root     root             0 Nov 14 23:19 ..
     -rw-r--r--    1 root     root             0 Nov 14 23:19 enable
     -rw-r--r--    1 root     root             0 Nov 14 23:19 filter
     -r--r--r--    1 root     root             0 Nov 14 23:19 format
     -r--r--r--    1 root     root             0 Nov 14 23:19 id
            </literallayout>
            The 'format' file for the tracepoint describes the event
            in memory, which is used by the various tracing tools
            that now make use of these tracepoint to parse the event
            and make sense of it, along with a 'print fmt' field that
            allows tools like ftrace to display the event as text.
            Here's what the format of the kmalloc event looks like:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# cat format
     name: kmalloc
     ID: 313
     format:
	     field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	     field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	     field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	     field:int common_pid;	offset:4;	size:4;	signed:1;
	     field:int common_padding;	offset:8;	size:4;	signed:1;

	     field:unsigned long call_site;	offset:16;	size:8;	signed:0;
	     field:const void * ptr;	offset:24;	size:8;	signed:0;
	     field:size_t bytes_req;	offset:32;	size:8;	signed:0;
	     field:size_t bytes_alloc;	offset:40;	size:8;	signed:0;
	     field:gfp_t gfp_flags;	offset:48;	size:4;	signed:0;

     print fmt: "call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s", REC->call_site, REC->ptr, REC->bytes_req, REC->bytes_alloc,
     (REC->gfp_flags) ? __print_flags(REC->gfp_flags, "|", {(unsigned long)(((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
     gfp_t)0x20000u) | (( gfp_t)0x02u) | (( gfp_t)0x08u)) | (( gfp_t)0x4000u) | (( gfp_t)0x10000u) | (( gfp_t)0x1000u) | (( gfp_t)0x200u) | ((
     gfp_t)0x400000u)), "GFP_TRANSHUGE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x20000u) | ((
     gfp_t)0x02u) | (( gfp_t)0x08u)), "GFP_HIGHUSER_MOVABLE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
     gfp_t)0x20000u) | (( gfp_t)0x02u)), "GFP_HIGHUSER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
     gfp_t)0x20000u)), "GFP_USER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x80000u)), GFP_TEMPORARY"},
     {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u)), "GFP_KERNEL"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u)),
     "GFP_NOFS"}, {(unsigned long)((( gfp_t)0x20u)), "GFP_ATOMIC"}, {(unsigned long)((( gfp_t)0x10u)), "GFP_NOIO"}, {(unsigned long)((
     gfp_t)0x20u), "GFP_HIGH"}, {(unsigned long)(( gfp_t)0x10u), "GFP_WAIT"}, {(unsigned long)(( gfp_t)0x40u), "GFP_IO"}, {(unsigned long)((
     gfp_t)0x100u), "GFP_COLD"}, {(unsigned long)(( gfp_t)0x200u), "GFP_NOWARN"}, {(unsigned long)(( gfp_t)0x400u), "GFP_REPEAT"}, {(unsigned
     long)(( gfp_t)0x800u), "GFP_NOFAIL"}, {(unsigned long)(( gfp_t)0x1000u), "GFP_NORETRY"},      {(unsigned long)(( gfp_t)0x4000u), "GFP_COMP"},
     {(unsigned long)(( gfp_t)0x8000u), "GFP_ZERO"}, {(unsigned long)(( gfp_t)0x10000u), "GFP_NOMEMALLOC"}, {(unsigned long)(( gfp_t)0x20000u),
     "GFP_HARDWALL"}, {(unsigned long)(( gfp_t)0x40000u), "GFP_THISNODE"}, {(unsigned long)(( gfp_t)0x80000u), "GFP_RECLAIMABLE"}, {(unsigned
     long)(( gfp_t)0x08u), "GFP_MOVABLE"}, {(unsigned long)(( gfp_t)0), "GFP_NOTRACK"}, {(unsigned long)(( gfp_t)0x400000u), "GFP_NO_KSWAPD"},
     {(unsigned long)(( gfp_t)0x800000u), "GFP_OTHER_NODE"} ) : "GFP_NOWAIT"
            </literallayout>
            The 'enable' file in the tracepoint directory is what allows
            the user (or tools such as trace-cmd) to actually turn the
            tracepoint on and off. When enabled, the corresponding
            tracepoint will start appearing in the ftrace 'trace'
            file described previously. For example, this turns on the
            kmalloc tracepoint:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 1 > enable
            </literallayout>
            At the moment, we're not interested in the function tracer or
            some other tracer that might be in effect, so we first turn
            it off, but if we do that, we still need to turn tracing on in
            order to see the events in the output buffer:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# echo nop > current_tracer
     root@sugarbay:/sys/kernel/debug/tracing# echo 1 > tracing_on
            </literallayout>
            Now, if we look at the the 'trace' file, we see nothing
            but the kmalloc events we just turned on:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing# cat trace | less
     # tracer: nop
     #
     # entries-in-buffer/entries-written: 1897/1897   #P:8
     #
     #                              _-----=&gt; irqs-off
     #                             / _----=&gt; need-resched
     #                            | / _---=&gt; hardirq/softirq
     #                            || / _--=&gt; preempt-depth
     #                            ||| /     delay
     #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
     #              | |       |   ||||       |         |
            dropbear-1465  [000] ...1 18154.620753: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
              &lt;idle&gt;-0     [000] ..s3 18154.621640: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
              &lt;idle&gt;-0     [000] ..s3 18154.621656: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
     matchbox-termin-1361  [001] ...1 18154.755472: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f0e00 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT
                Xorg-1264  [002] ...1 18154.755581: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
                Xorg-1264  [002] ...1 18154.755583: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
                Xorg-1264  [002] ...1 18154.755589: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
     matchbox-termin-1361  [001] ...1 18155.354594: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db35400 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT
                Xorg-1264  [002] ...1 18155.354703: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
                Xorg-1264  [002] ...1 18155.354705: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
                Xorg-1264  [002] ...1 18155.354711: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
              &lt;idle&gt;-0     [000] ..s3 18155.673319: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
            dropbear-1465  [000] ...1 18155.673525: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
              &lt;idle&gt;-0     [000] ..s3 18155.674821: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
              &lt;idle&gt;-0     [000] ..s3 18155.793014: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
            dropbear-1465  [000] ...1 18155.793219: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
              &lt;idle&gt;-0     [000] ..s3 18155.794147: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
              &lt;idle&gt;-0     [000] ..s3 18155.936705: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
            dropbear-1465  [000] ...1 18155.936910: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
              &lt;idle&gt;-0     [000] ..s3 18155.937869: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
     matchbox-termin-1361  [001] ...1 18155.953667: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f2000 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT
                Xorg-1264  [002] ...1 18155.953775: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
                Xorg-1264  [002] ...1 18155.953777: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
                Xorg-1264  [002] ...1 18155.953783: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
              &lt;idle&gt;-0     [000] ..s3 18156.176053: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
            dropbear-1465  [000] ...1 18156.176257: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
              &lt;idle&gt;-0     [000] ..s3 18156.177717: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
              &lt;idle&gt;-0     [000] ..s3 18156.399229: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
            dropbear-1465  [000] ...1 18156.399434: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_http://rostedt.homelinux.com/kernelshark/req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
              &lt;idle&gt;-0     [000] ..s3 18156.400660: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
     matchbox-termin-1361  [001] ...1 18156.552800: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db34800 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT
            </literallayout>
            To again disable the kmalloc event, we need to send 0 to the
            enable file:
            <literallayout class='monospaced'>
     root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 0 > enable
            </literallayout>
            You can enable any number of events or complete subsystems
            (by using the 'enable' file in the subsystem directory) and
            get am arbitrarily fine-grained idea of what's going on in the
            system by enabling as many of the appropriate tracepoints
            as applicable.
        </para>

        <para>
            A number of the tools described in this HOWTO do just that,
            including trace-cmd and kernelshark in the next section.
        </para>

        <note>
            Tying It Together: These tracepoints and their representation
            are used not only by ftrace,  but by many of the other tools
            covered in this document and they form a central point of
            integration for the various tracers available in Linux.
            They form a central part of the instrumentation for the
            following tools: perf, lttng, ftrace, blktrace and SystemTap
        </note>

        <note>
            Tying It Together: Eventually all the special-purpose tracers
            currently available in /sys/kernel/debug/tracing will be
            removed and replaced with equivalent tracers based on the
            'trace events' subsystem.
        </note>
    </section>

    <section id='trace-cmd-kernelshark'>
        <title>trace-cmd/kernelshark</title>

        <para>
            trace-cmd is essentially an extensive command-line 'wrapper'
            interface that hides the details of all the individual files
            in /sys/kernel/debug/tracing, allowing users to specify
            specific particular events within the
            /sys/kernel/debug/tracing/events/ subdirectory and to collect
            traces and avoiding having to deal with those details directly.
        </para>

        <para>
            As yet another layer on top of that, kernelshark provides a GUI
            that allows users to start and stop traces and specify sets
            of events using an intuitive interface, and view the
            output as both trace events and as a per-cpu graphical
            display. It directly uses 'trace-cmd' as the plumbing
            that accomplishes all that underneath the covers (and
            actually displays the trace-cmd command it uses, as we'll see).
        </para>

        <para>
            To start a trace using kernelshark, first start kernelshark:
            <literallayout class='monospaced'>
     root@sugarbay:~# kernelshark
            </literallayout>
            The bring up the 'Capture' dialog by choosing from the
            kernelshark menu:
            <literallayout class='monospaced'>
     Capture | Record
            </literallayout>
            That will display the following dialog, which allows you to
            choose on or more events (or even one or more complete
            subsystems) to trace:
        </para>

        <para>
            <imagedata fileref="figures/kernelshark-choose-events.png" width="6in" depth="7in" align="center" scalefit="1" />
        </para>

        <para>
            Note that these are exactly the same set of events described
            in the previous trace events subsystem section, and in fact
            is where trace-cmd gets them for kernelshark.
        </para>

        <para>
            In the above screenshot, we've decided to explore the
            graphics subsystem a bit and so have chosen to trace all
            the tracepoints contained within the 'i915' and 'drm'
            subsystems.
        </para>

        <para>
            After doing that, we can start and stop the trace using
            the 'Run' and 'Stop' button on the lower right corner of
            the dialog (the same button will turn into the 'Stop'
            button after the trace has started):
        </para>

        <para>
            <imagedata fileref="figures/kernelshark-output-display.png" width="6in" depth="7in" align="center" scalefit="1" />
        </para>

        <para>
            Notice that the right-hand pane shows the exact trace-cmd
            command-line that's used to run the trace, along with the
            results of the trace-cmd run.
        </para>

        <para>
            Once the 'Stop' button is pressed, the graphical view magically
            fills up with a colorful per-cpu display of the trace data,
            along with the detailed event listing below that:
        </para>

        <para>
            <imagedata fileref="figures/kernelshark-i915-display.png" width="6in" depth="7in" align="center" scalefit="1" />
        </para>

        <para>
            Here's another example, this time a display resulting
            from tracing 'all events':
        </para>

        <para>
            <imagedata fileref="figures/kernelshark-all.png" width="6in" depth="7in" align="center" scalefit="1" />
        </para>

        <para>
            The tool is pretty self-explanatory, but for more detailed
            information on navigating through the data, see the
            <ulink url='http://rostedt.homelinux.com/kernelshark/'>kernelshark website</ulink>.
        </para>
    </section>

    <section id='ftrace-documentation'>
        <title>Documentation</title>

        <para>
            The documentation for ftrace can be found in the kernel
            Documentation directory:
            <literallayout class='monospaced'>
     Documentation/trace/ftrace.txt
            </literallayout>
            The documentation for the trace event subsystem can also
            be found in the kernel Documentation directory:
            <literallayout class='monospaced'>
     Documentation/trace/events.txt
            </literallayout>
            There are a nice series of articles on using
            ftrace and trace-cmd at LWN:
            <itemizedlist>
                <listitem><para><ulink url='http://lwn.net/Articles/365835/'>Debugging the kernel using Ftrace - part 1</ulink>
                    </para></listitem>
                <listitem><para><ulink url='http://lwn.net/Articles/366796/'>Debugging the kernel using Ftrace - part 2</ulink>
                    </para></listitem>
                <listitem><para><ulink url='https://lwn.net/Articles/410200/'>trace-cmd: A front-end for Ftrace</ulink>
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            There's more detailed documentation kernelshark usage here:
            <ulink url='http://rostedt.homelinux.com/kernelshark/'>KernelShark</ulink>
        </para>

        <para>
            An amusing yet useful README (a tracing mini-HOWTO) can be
            found in /sys/kernel/debug/tracing/README.
        </para>
    </section>
</section>

</chapter>
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