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perf/core: Skip empty AUX records with only format flags

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perf_aux_output_end() emits a PERF_RECORD_AUX when the recorded size is
nonzero or when any flag other than PERF_AUX_FLAG_OVERWRITE is set.

PMU format flags describe how an AUX payload is encoded. TRBE driver
sets PERF_AUX_FLAG_CORESIGHT_FORMAT_RAW for raw trace buffers, causing
an AUX record to be emitted even when no trace data.

This is noticeable when tracing a task with strace. Ptrace stops
repeatedly end empty AUX transactions, producing many zero-sized
PERF_RECORD_AUX records. For example:

  perf record -e cs_etm//u -m,128M -- strace ls

  perf script -D 2>&1 |
     awk '/PERF_RECORD_AUX offset/ {
        for (i = 1; i <= NF; i++)
                if ($i == "size:" && $(i + 1) == "0")
                        count++
     }
     END { print count }'
  165

This recording contains 165 zero-sized AUX records which provide no
useful information to userspace.

Ignore PERF_AUX_FLAG_PMU_FORMAT_TYPE_MASK, together with
PERF_AUX_FLAG_OVERWRITE, when deciding whether an empty AUX record is
useful. Zero-sized records carrying TRUNCATED, PARTIAL or COLLISION
are still emitted.

Fixes: 547b60988e ("perf: aux: Add flags for the buffer format")
Reported-by: Tamas Petz <tamas.petz@arm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260825-perf_core_fix_zero_aux_records-v1-1-23b95e8d5df3@arm.com
This commit is contained in:
Leo Yan 2026-08-25 11:37:06 +01:00 committed by Peter Zijlstra
parent cee9395acd
commit 8a7f5b5e86
1 changed files with 7 additions and 2 deletions
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9
kernel/events/ring_buffer.c
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@ -509,7 +509,10 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
/*
* Only send RECORD_AUX if we have something useful to communicate
*
* Note: the OVERWRITE records by themselves are not considered
* PMU_FORMAT bits identify the PMU type rather than an AUX event
* has occurred, so ignore them for zero-sized records.
*
* 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.
@ -518,7 +521,9 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
* offset. So, from now on we don't output AUX records that
* have *only* OVERWRITE flag set.
*/
if (size || (handle->aux_flags & ~(u64)PERF_AUX_FLAG_OVERWRITE))
if (size ||
(handle->aux_flags & ~(u64)(PERF_AUX_FLAG_PMU_FORMAT_TYPE_MASK |
PERF_AUX_FLAG_OVERWRITE)))
perf_event_aux_event(handle->event, aux_head, size,
handle->aux_flags);