tracing: Add hist trigger onchange() handler Documentation
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>
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Tom Zanussi
Steven Rostedt (VMware)
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@ -1869,6 +1869,7 @@ The available handlers are:
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- onmatch(matching.event) - invoke action on any addition or update
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- onmax(var) - invoke action if var exceeds current max
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- onchange(var) - invoke action if var changes
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The available actions are:
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@ -2140,6 +2141,103 @@ The following commonly-used handler.action pairs are available:
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<...>-2102 [005] d..4 309.875185: sched_wake_idle_without_ipi: cpu=1
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<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
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- onchange(var).save(field,.. .)
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The 'onchange(var).save(field,...)' hist trigger action is invoked
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whenever the value of 'var' associated with a histogram entry
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changes.
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The end result is that the trace event fields specified as the
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onchange.save() params will be saved if 'var' changes for that
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hist trigger entry. This allows context from the event that
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changed the value to be saved for later reference. When the
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histogram is displayed, additional fields displaying the saved
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values will be printed.
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- onchange(var).snapshot()
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The 'onchange(var).snapshot()' hist trigger action is invoked
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whenever the value of 'var' associated with a histogram entry
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changes.
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The end result is that a global snapshot of the trace buffer will
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be saved in the tracing/snapshot file if 'var' changes for any
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hist trigger entry.
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Note that in this case the changed value is a global variable
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associated withe current trace instance. The key of the specific
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trace event that caused the value to change and the global value
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itself are displayed, along with a message stating that a snapshot
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has been taken and where to find it. The user can use the key
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information displayed to locate the corresponding bucket in the
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histogram for even more detail.
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As an example the below defines a hist trigger on the tcp_probe
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event, keyed on dport. Whenever a tcp_probe event occurs, the
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cwnd field is checked against the current value stored in the
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$cwnd variable. If the value has changed, a snapshot is taken.
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As part of the setup, all the scheduler and tcp events are also
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enabled, which are the events that will show up in the snapshot
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when it is taken at some point:
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# echo 1 > /sys/kernel/debug/tracing/events/sched/enable
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# echo 1 > /sys/kernel/debug/tracing/events/tcp/enable
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# echo 'hist:keys=dport:cwnd=snd_cwnd: \
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onchange($cwnd).save(snd_wnd,srtt,rcv_wnd): \
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onchange($cwnd).snapshot()' >> \
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/sys/kernel/debug/tracing/events/tcp/tcp_probe/trigger
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When the histogram is displayed, for each bucket the tracked value
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and the saved values corresponding to that value are displayed
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following the rest of the fields.
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If a snaphot was taken, there is also a message indicating that,
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along with the value and event that triggered the snapshot:
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# cat /sys/kernel/debug/tracing/events/tcp/tcp_probe/hist
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{ dport: 1521 } hitcount: 8
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changed: 10 snd_wnd: 35456 srtt: 154262 rcv_wnd: 42112
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{ dport: 80 } hitcount: 23
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changed: 10 snd_wnd: 28960 srtt: 19604 rcv_wnd: 29312
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{ dport: 9001 } hitcount: 172
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changed: 10 snd_wnd: 48384 srtt: 260444 rcv_wnd: 55168
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{ dport: 443 } hitcount: 211
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changed: 10 snd_wnd: 26960 srtt: 17379 rcv_wnd: 28800
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Snapshot taken (see tracing/snapshot). Details:
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triggering value { onchange($cwnd) }: 10
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triggered by event with key: { dport: 80 }
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Totals:
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Hits: 414
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Entries: 4
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Dropped: 0
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In the above case, the event that triggered the snapshot has the
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key with dport == 80. If you look at the bucket that has 80 as
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the key, you'll find the additional values save()'d along with the
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changed value for that bucket, which should be the same as the
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global changed value (since that was the same value that triggered
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the global snapshot).
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And finally, looking at the snapshot data should show at or near
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the end the event that triggered the snapshot:
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# cat /sys/kernel/debug/tracing/snapshot
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gnome-shell-1261 [006] dN.3 49.823113: sched_stat_runtime: comm=gnome-shell pid=1261 runtime=49347 [ns] vruntime=1835730389 [ns]
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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
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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
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kworker/3:2-135 [003] d..3 49.823118: sched_stat_runtime: comm=kworker/3:2 pid=135 runtime=5339 [ns] vruntime=17815800388 [ns]
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kworker/6:2-387 [006] d..3 49.823120: sched_stat_runtime: comm=kworker/6:2 pid=387 runtime=9594 [ns] vruntime=14589605367 [ns]
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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
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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
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<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
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3. User space creating a trigger
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--------------------------------
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