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/...
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