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Subject[PATCH 0/5] tracing: Extend histogram triggers expression parsing
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The frequency of the rss_stat trace event is known to be of the same
magnitude as that of the sched_switch event on Android devices. This can
cause flooding of the trace buffer with rss_stat traces leading to a
decreased trace buffer capacity and loss of data.

If it is not necessary to monitor very small changes in rss (as is the
case in Android) then the rss_stat tracepoint can be throttled to only
emit the event once there is a large enough change in the rss size.
The original patch that introduced the rss_stat tracepoint also proposed
a fixed throttling mechanism that only emits the rss_stat event
when the rss size crosses a 512KB boundary. It was concluded that more
generic support for this type of filtering/throttling was need, so that
it can be applied to any trace event. [1]

From the discussion in [1], histogram triggers seemed the most likely
candidate to support this type of throttling. For instance to achieve the
same throttling as was proposed in [1]:

(1) Create a histogram variable to save the 512KB bucket of the rss size
(2) Use the onchange handler to generate a synthetic event when the
rss size bucket changes.

The only missing pieces to support such a hist trigger are:
(1) Support for setting a hist variable to a specific value -- to set
the bucket size / granularity.
(2) Support for division arithmetic operation -- to determine the
corresponding bucket for an rss size.

This series extends histogram trigger expressions to:
(1) Allow assigning numeric literals to hist variable (eg. x=1234)
and using literals directly in expressions (eg. x=size/1234)
(2) Support division and multiplication in hist expressions.
(eg. a=$x/$y*z); and
(3) Fixes expression parsing for non-associative operators: subtraction
and division. (eg. 8-4-2 should be 2 not 6)

The rss_stat event can then be throttled using histogram triggers as
below:

# Create a synthetic event to monitor instead of the high frequency
# rss_stat event
echo 'rss_stat_throttled unsigned int mm_id; unsigned int curr;
int member; long size' >> tracing/synthetic_events

# Create a hist trigger that emits the synthetic rss_stat_throttled
# event only when the rss size crosses a 512KB boundary.
echo 'hist:keys=common_pid:bucket=size/0x80000:onchange($bucket)
.rss_stat_throttled(mm_id,curr,member,size)'
>> events/kmem/rss_stat/trigger

------ Test Results ------
Histograms can also be used to evaluate the effectiveness of this
throttling by noting the Total Hits on each trigger:

echo 'hist:keys=common_pid' >> events/sched/sched_switch/trigger
echo 'hist:keys=common_pid' >> events/kmem/rss_stat/trigger
echo 'hist:keys=common_pid'
>> events/synthetic/rss_stat_throttled/trigger

Allowing the above example (512KB granularity) run for 5 minutes on
an arm64 device with 5.10 kernel:

sched_switch : total hits = 147153
rss_stat : total hits = 38863
rss_stat_throttled: total hits = 2409

The synthetic rss_stat_throttled event is ~16x less frequent than the
rss_stat event when using a 512KB granularity.


The results are more pronounced when rss size is changing at a higher
rate in small increments. For instance the following results were obtained
by recording the hits on the above events for a run of Android's
lmkd_unit_test [2], which continually forks processes that map anonymous
memory until there is an oom kill:

sched_switch : total hits = 148832
rss_stat : total hits = 4754802
rss_stat_throttled: total hits = 96214

In this stress this, the synthetic rss_stat_throttled event is ~50x less
frequent than the rss_stat event when using a 512KB granularity.


[1] https://lore.kernel.org/lkml/20190903200905.198642-1-joel@joelfernandes.org/
[2] https://cs.android.com/android/platform/superproject/+/master:system/memory/lmkd/tests/lmkd_test.cpp

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

Kalesh Singh (5):
tracing: Add support for creating hist trigger variables from literal
tracing: Add division and multiplication support for hist triggers
tracing: Fix operator precedence for hist triggers expression
tracing/selftests: Add tests for hist trigger expression parsing
tracing/histogram: Document expression arithmetic and constants

Documentation/trace/histogram.rst | 14 +
kernel/trace/trace_events_hist.c | 318 +++++++++++++++---
.../testing/selftests/ftrace/test.d/functions | 4 +-
.../trigger/trigger-hist-expressions.tc | 73 ++++
4 files changed, 357 insertions(+), 52 deletions(-)
create mode 100644 tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-expressions.tc


base-commit: 3ca706c189db861b2ca2019a0901b94050ca49d8
--
2.33.0.309.g3052b89438-goog

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