perf/aux: Only update ->aux_wakeup in non-overwrite mode (441430eb) · Commits · 戴 / test

The following commit: ("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>

kernel/events/ring_buffer.c

+15 −5

Original line number	Diff line number	Diff line
		@@ -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;
		}

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