[PATCH v7 08/16] tracing: Add hist trigger snapshot() action Documentation

Tom Zanussi
2018-11-14T20:18:35Z

From: Tom Zanussi <tom.zanussi@linux.intel.com>

Add Documentation for the hist:handlerXXX($var).snapshot() action.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
---
Documentation/trace/histogram.rst | 110 ++++++++++++++++++++++++++++++++++++++
1 file changed, 110 insertions(+)

diff --git a/Documentation/trace/histogram.rst b/Documentation/trace/histogram.rst
index 63e522107e59..353317bc3825 100644
--- a/Documentation/trace/histogram.rst
+++ b/Documentation/trace/histogram.rst
@@ -1874,6 +1874,7 @@ The available actions are:

- <synthetic_event_name>(param list) - generate synthetic event
- save(field,...) - save current event fields
+ - snapshot() - snapshot the trace buffer

The following commonly-used handler.action pairs are available:

@@ -2030,6 +2031,115 @@ The following commonly-used handler.action pairs are available:
Entries: 2
Dropped: 0

+ - onmax(var).snapshot()
+
+ The 'onmax(var).snapshot()' hist trigger action is invoked
+ whenever the value of 'var' associated with a histogram entry
+ exceeds the current maximum contained in that variable.
+
+ The end result is that a global snapshot of the trace buffer will
+ be saved in the tracing/snapshot file if 'var' exceeds the current
+ maximum for any hist trigger entry.
+
+ Note that in this case the maximum is a global maximum for the
+ current trace instance, which is the maximum across all buckets of
+ the histogram. The key of the specific trace event that caused
+ the global maximum and the global maximum 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 couple of hist triggers, one for
+ sched_waking and another for sched_switch, keyed on pid. Whenever
+ a sched_waking event occurs, the timestamp is saved in the entry
+ corresponding to the current pid, and when the scheduler switches
+ back to that pid, the timestamp difference is calculated. If the
+ resulting latency, stored in wakeup_lat, exceeds the current
+ maximum latency, a snapshot is taken. As part of the setup, all
+ the scheduler 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 'hist:keys=pid:ts0=common_timestamp.usecs \
+ if comm=="cyclictest"' >> \
+ /sys/kernel/debug/tracing/events/sched/sched_waking/trigger
+
+ # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
+ onmax($wakeup_lat).save(next_prio,next_comm,prev_pid,prev_prio, \
+ prev_comm):onmax($wakeup_lat).snapshot() \
+ if next_comm=="cyclictest"' >> \
+ /sys/kernel/debug/tracing/events/sched/sched_switch/trigger
+
+ When the histogram is displayed, for each bucket the max value
+ and the saved values corresponding to the max are displayed
+ following the rest of the fields.
+
+ If a snaphot was taken, there is also a message indicating that,
+ along with the value and event that triggered the global maximum:
+
+ # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
+ { next_pid: 2101 } hitcount: 200
+ max: 52 next_prio: 120 next_comm: cyclictest \
+ prev_pid: 0 prev_prio: 120 prev_comm: swapper/6
+
+ { next_pid: 2103 } hitcount: 1326
+ max: 572 next_prio: 19 next_comm: cyclictest \
+ prev_pid: 0 prev_prio: 120 prev_comm: swapper/1
+
+ { next_pid: 2102 } hitcount: 1982 \
+ max: 74 next_prio: 19 next_comm: cyclictest \
+ prev_pid: 0 prev_prio: 120 prev_comm: swapper/5
+
+ Snapshot taken (see tracing/snapshot). Details:
+ triggering value { onmax($wakeup_lat) }: 572 \
+ triggered by event with key: { next_pid: 2103 }
+
+ Totals:
+ Hits: 3508
+ Entries: 3
+ Dropped: 0
+
+ In the above case, the event that triggered the global maximum has
+ the key with next_pid == 2103. If you look at the bucket that has
+ 2103 as the key, you'll find the additional values save()'d along
+ with the local maximum for that bucket, which should be the same
+ as the global maximum (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 (in this case you
+ can verify the timestamps between the sched_waking and
+ sched_switch events, which should match the time displayed in the
+ global maximum):
+
+ # cat /sys/kernel/debug/tracing/snapshot
+
+ <...>-2103 [005] d..3 309.873125: sched_switch: prev_comm=cyclictest prev_pid=2103 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
+ <idle>-0 [005] d.h3 309.873611: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
+ <idle>-0 [005] dNh4 309.873613: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
+ <idle>-0 [005] d..3 309.873616: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
+ <...>-2102 [005] d..3 309.873625: sched_switch: prev_comm=cyclictest prev_pid=2102 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
+ <idle>-0 [005] d.h3 309.874624: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
+ <idle>-0 [005] dNh4 309.874626: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
+ <idle>-0 [005] dNh3 309.874628: sched_waking: comm=cyclictest pid=2103 prio=19 target_cpu=005
+ <idle>-0 [005] dNh4 309.874630: sched_wakeup: comm=cyclictest pid=2103 prio=19 target_cpu=005
+ <idle>-0 [005] d..3 309.874633: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
+ <idle>-0 [004] d.h3 309.874757: sched_waking: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
+ <idle>-0 [004] dNh4 309.874762: sched_wakeup: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
+ <idle>-0 [004] d..3 309.874766: sched_switch: prev_comm=swapper/4 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=gnome-terminal- next_pid=1699 next_prio=120
+ gnome-terminal--1699 [004] d.h2 309.874941: sched_stat_runtime: comm=gnome-terminal- pid=1699 runtime=180706 [ns] vruntime=1126870572 [ns]
+ <idle>-0 [003] d.s4 309.874956: sched_waking: comm=rcu_sched pid=9 prio=120 target_cpu=007
+ <idle>-0 [003] d.s5 309.874960: sched_wake_idle_without_ipi: cpu=7
+ <idle>-0 [003] d.s5 309.874961: sched_wakeup: comm=rcu_sched pid=9 prio=120 target_cpu=007
+ <idle>-0 [007] d..3 309.874963: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=rcu_sched next_pid=9 next_prio=120
+ rcu_sched-9 [007] d..3 309.874973: sched_stat_runtime: comm=rcu_sched pid=9 runtime=13646 [ns] vruntime=22531430286 [ns]
+ rcu_sched-9 [007] d..3 309.874978: sched_switch: prev_comm=rcu_sched prev_pid=9 prev_prio=120 prev_state=R+ ==> next_comm=swapper/7 next_pid=0 next_prio=120
+ <...>-2102 [005] d..4 309.874994: sched_migrate_task: comm=cyclictest pid=2103 prio=19 orig_cpu=5 dest_cpu=1
+ <...>-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
+
3. User space creating a trigger
--------------------------------

--
2.14.1