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    Subject[for-next][PATCH 22/29] tracing: Add hist trigger onchange() handler Documentation
    From: Tom Zanussi <tom.zanussi@linux.intel.com>

    Add Documentation for the hist:onchange($var) handler.

    Link: http://lkml.kernel.org/r/ab54b7383b265609fda52648a8fbfbd2631a640f.1550100284.git.tom.zanussi@linux.intel.com

    Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    ---
    Documentation/trace/histogram.rst | 98 +++++++++++++++++++++++++++++++
    1 file changed, 98 insertions(+)

    diff --git a/Documentation/trace/histogram.rst b/Documentation/trace/histogram.rst
    index 353317bc3825..79476c906b1a 100644
    --- a/Documentation/trace/histogram.rst
    +++ b/Documentation/trace/histogram.rst
    @@ -1869,6 +1869,7 @@ The available handlers are:

    - onmatch(matching.event) - invoke action on any addition or update
    - onmax(var) - invoke action if var exceeds current max
    + - onchange(var) - invoke action if var changes

    The available actions are:

    @@ -2140,6 +2141,103 @@ The following commonly-used handler.action pairs are available:
    <...>-2102 [005] d..4 309.875185: sched_wake_idle_without_ipi: cpu=1
    <idle>-0 [001] d..3 309.875200: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2103 next_prio=19

    + - onchange(var).save(field,.. .)
    +
    + The 'onchange(var).save(field,...)' hist trigger action is invoked
    + whenever the value of 'var' associated with a histogram entry
    + changes.
    +
    + The end result is that the trace event fields specified as the
    + onchange.save() params will be saved if 'var' changes for that
    + hist trigger entry. This allows context from the event that
    + changed the value to be saved for later reference. When the
    + histogram is displayed, additional fields displaying the saved
    + values will be printed.
    +
    + - onchange(var).snapshot()
    +
    + The 'onchange(var).snapshot()' hist trigger action is invoked
    + whenever the value of 'var' associated with a histogram entry
    + changes.
    +
    + The end result is that a global snapshot of the trace buffer will
    + be saved in the tracing/snapshot file if 'var' changes for any
    + hist trigger entry.
    +
    + Note that in this case the changed value is a global variable
    + associated withe current trace instance. The key of the specific
    + trace event that caused the value to change and the global value
    + itself are displayed, along with a message stating that a snapshot
    + has been taken and where to find it. The user can use the key
    + information displayed to locate the corresponding bucket in the
    + histogram for even more detail.
    +
    + As an example the below defines a hist trigger on the tcp_probe
    + event, keyed on dport. Whenever a tcp_probe event occurs, the
    + cwnd field is checked against the current value stored in the
    + $cwnd variable. If the value has changed, a snapshot is taken.
    + As part of the setup, all the scheduler and tcp events are also
    + enabled, which are the events that will show up in the snapshot
    + when it is taken at some point:
    +
    + # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
    + # echo 1 > /sys/kernel/debug/tracing/events/tcp/enable
    +
    + # echo 'hist:keys=dport:cwnd=snd_cwnd: \
    + onchange($cwnd).save(snd_wnd,srtt,rcv_wnd): \
    + onchange($cwnd).snapshot()' >> \
    + /sys/kernel/debug/tracing/events/tcp/tcp_probe/trigger
    +
    + When the histogram is displayed, for each bucket the tracked value
    + and the saved values corresponding to that value are displayed
    + following the rest of the fields.
    +
    + If a snaphot was taken, there is also a message indicating that,
    + along with the value and event that triggered the snapshot:
    +
    + # cat /sys/kernel/debug/tracing/events/tcp/tcp_probe/hist
    + { dport: 1521 } hitcount: 8
    + changed: 10 snd_wnd: 35456 srtt: 154262 rcv_wnd: 42112
    +
    + { dport: 80 } hitcount: 23
    + changed: 10 snd_wnd: 28960 srtt: 19604 rcv_wnd: 29312
    +
    + { dport: 9001 } hitcount: 172
    + changed: 10 snd_wnd: 48384 srtt: 260444 rcv_wnd: 55168
    +
    + { dport: 443 } hitcount: 211
    + changed: 10 snd_wnd: 26960 srtt: 17379 rcv_wnd: 28800
    +
    + Snapshot taken (see tracing/snapshot). Details:
    + triggering value { onchange($cwnd) }: 10
    + triggered by event with key: { dport: 80 }
    +
    + Totals:
    + Hits: 414
    + Entries: 4
    + Dropped: 0
    +
    + In the above case, the event that triggered the snapshot has the
    + key with dport == 80. If you look at the bucket that has 80 as
    + the key, you'll find the additional values save()'d along with the
    + changed value for that bucket, which should be the same as the
    + global changed value (since that was the same value that triggered
    + the global snapshot).
    +
    + And finally, looking at the snapshot data should show at or near
    + the end the event that triggered the snapshot:
    +
    + # cat /sys/kernel/debug/tracing/snapshot
    +
    + gnome-shell-1261 [006] dN.3 49.823113: sched_stat_runtime: comm=gnome-shell pid=1261 runtime=49347 [ns] vruntime=1835730389 [ns]
    + kworker/u16:4-773 [003] d..3 49.823114: sched_switch: prev_comm=kworker/u16:4 prev_pid=773 prev_prio=120 prev_state=R+ ==> next_comm=kworker/3:2 next_pid=135 next_prio=120
    + gnome-shell-1261 [006] d..3 49.823114: sched_switch: prev_comm=gnome-shell prev_pid=1261 prev_prio=120 prev_state=R+ ==> next_comm=kworker/6:2 next_pid=387 next_prio=120
    + kworker/3:2-135 [003] d..3 49.823118: sched_stat_runtime: comm=kworker/3:2 pid=135 runtime=5339 [ns] vruntime=17815800388 [ns]
    + kworker/6:2-387 [006] d..3 49.823120: sched_stat_runtime: comm=kworker/6:2 pid=387 runtime=9594 [ns] vruntime=14589605367 [ns]
    + kworker/6:2-387 [006] d..3 49.823122: sched_switch: prev_comm=kworker/6:2 prev_pid=387 prev_prio=120 prev_state=R+ ==> next_comm=gnome-shell next_pid=1261 next_prio=120
    + kworker/3:2-135 [003] d..3 49.823123: sched_switch: prev_comm=kworker/3:2 prev_pid=135 prev_prio=120 prev_state=T ==> next_comm=swapper/3 next_pid=0 next_prio=120
    + <idle>-0 [004] ..s7 49.823798: tcp_probe: src=10.0.0.10:54326 dest=23.215.104.193:80 mark=0x0 length=32 snd_nxt=0xe3ae2ff5 snd_una=0xe3ae2ecd snd_cwnd=10 ssthresh=2147483647 snd_wnd=28960 srtt=19604 rcv_wnd=29312
    +
    3. User space creating a trigger
    --------------------------------

    --
    2.20.1

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