On Wed, Sep 15, 2021 at 12:53 PM Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote: > > 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 > Sorry, I have a clerical mistake here. The above key should be: s/keys=common_pid/keys=keys=mm_id,member The rss size is specific to the mm struct's member not the pid. The results below were captured with the correct key so no changes there. > ------ 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@xxxxxxxxxxxxxxxxx/ > [2] https://cs.android.com/android/platform/superproject/+/master:system/memory/lmkd/tests/lmkd_test.cpp > > Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx> > > 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 >
