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

[Kalesh Singh <kaleshsingh@google.com>]

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>
>
> 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
>