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perf/aux: Only update ->aux_wakeup in non-overwrite mode

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The following commit:

  d9a50b0256 ("perf/aux: Ensure aux_wakeup represents most recent wakeup index")

changed the AUX wakeup position calculation to rounddown(), which causes
a division-by-zero in AUX overwrite mode (aka "snapshot mode").

The zero denominator results from the fact that perf record doesn't set
aux_watermark to anything, in which case the kernel will set it to half
the AUX buffer size, but only for non-overwrite mode. In the overwrite
mode aux_watermark stays zero.

The good news is that, AUX overwrite mode, wakeups don't happen and
related bookkeeping is not relevant, so we can simply forego the whole
wakeup updates.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/20170906160811.16510-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Alexander Shishkin 2017-09-06 19:08:11 +03:00 committed by Ingo Molnar
parent 770b782f55
commit 441430eb54
1 changed files with 15 additions and 5 deletions
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20
kernel/events/ring_buffer.c
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@ -412,6 +412,19 @@ err:
return NULL;
}
static bool __always_inline rb_need_aux_wakeup(struct ring_buffer *rb)
{
if (rb->aux_overwrite)
return false;
if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) {
rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark);
return true;
}
return false;
}
/*
* Commit the data written by hardware into the ring buffer by adjusting
* aux_head and posting a PERF_RECORD_AUX into the perf buffer. It is the
@ -451,10 +464,8 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
}
rb->user_page->aux_head = rb->aux_head;
if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) {
if (rb_need_aux_wakeup(rb))
wakeup = true;
rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark);
}
if (wakeup) {
if (handle->aux_flags & PERF_AUX_FLAG_TRUNCATED)
@ -484,9 +495,8 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size)
rb->aux_head += size;
rb->user_page->aux_head = rb->aux_head;
if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) {
if (rb_need_aux_wakeup(rb)) {
perf_output_wakeup(handle);
rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark);
handle->wakeup = rb->aux_wakeup + rb->aux_watermark;
}