Re: [PATCH 0/5] tracing: Extend histogram triggers expression parsing

[Namhyung Kim <namhyung@kernel.org>]

Hi Kalesh,

On Wed, Sep 15, 2021 at 1:09 PM Kalesh Singh <kaleshsingh@google.com> wrote:
>
> On Wed, Sep 15, 2021 at 12:53 PM Kalesh Singh <kaleshsingh@google.com> 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@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>

Reviewed-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


> >
> > 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
> >