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authorAlexander Shishkin <alexander.shishkin@linux.intel.com>2018-04-04 17:53:23 +0300
committerArnaldo Carvalho de Melo <acme@redhat.com>2018-09-18 17:21:13 -0300
commit1627314fb54a33ebd23bd08f2e215eaed0f44712 (patch)
treea03f91adc007c7233eb2dc64641830a740ff9522 /kernel/events
parent7f16023bfcc4b1f2bbccf86219dc97473db6e0d6 (diff)
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>
Diffstat (limited to 'kernel/events')
-rw-r--r--kernel/events/ring_buffer.c14
1 files changed, 12 insertions, 2 deletions
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 5d3cf407e374..4a9937076331 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -459,10 +459,20 @@ 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.
*/
- perf_event_aux_event(handle->event, aux_head, size,
- handle->aux_flags);
+ if (handle->aux_flags & ~(u64)PERF_AUX_FLAG_OVERWRITE)
+ perf_event_aux_event(handle->event, aux_head, size,
+ handle->aux_flags);
}
rb->user_page->aux_head = rb->aux_head;