Commit 1627314f authored Apr 04, 2018 by Alexander Shishkin Committed by Arnaldo Carvalho de Melo Sep 18, 2018

perf: Suppress AUX/OVERWRITE records



It has been pointed out to me many times that it is useful to be able to
switch off AUX records to save the bandwidth for records that actually
matter, for example, in AUX overwrite mode.

The usefulness of PERF_RECORD_AUX is in some of its flags, like the
TRUNCATED flag that tells the decoder where exactly gaps in the trace
are.  The OVERWRITE flag, on the other hand will be set on every single
record in overwrite mode. However, a PERF_RECORD_AUX[flags=OVERWRITE] is
generated on every target task's sched_out, which over time adds up to a
lot of useless information.

If any folks out there have userspace that depends on a constant stream
of OVERWRITE records for a good reason, they'll have to let us know.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Link: http://lkml.kernel.org/r/20180404145323.28651-1-alexander.shishkin@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

parent 7f16023b

kernel/events/ring_buffer.c

+12 −2

Original line number	Diff line number	Diff line
		@@ -459,8 +459,18 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
		if (size \|\| handle->aux_flags) {
		/*
		* Only send RECORD_AUX if we have something useful to communicate
		*
		* Note: the OVERWRITE records by themselves are not considered
		* useful, as they don't communicate any new information,
		* aside from the short-lived offset, that becomes history at
		* the next event sched-in and therefore isn't useful.
		* The userspace that needs to copy out AUX data in overwrite
		* mode should know to use user_page::aux_head for the actual
		* offset. So, from now on we don't output AUX records that
		* have only OVERWRITE flag set.
		*/

		if (handle->aux_flags & ~(u64)PERF_AUX_FLAG_OVERWRITE)
		perf_event_aux_event(handle->event, aux_head, size,
		handle->aux_flags);
		}