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Commit 441430eb authored by Alexander Shishkin's avatar Alexander Shishkin Committed by Ingo Molnar
Browse files

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



The following commit:

  d9a50b02 ("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: default avatarAlexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent 770b782f

kernel/events/ring_buffer.c

+15 −5
Original line number Diff line number Diff line
@@ -412,6 +412,19 @@ void *perf_aux_output_begin(struct perf_output_handle *handle, 
	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;

	}