Commit 4ec8363d authored Jun 01, 2011 by Vince Weaver Committed by Ingo Molnar Jul 01, 2011

perf_events: Fix perf buffer watermark setting



Since 2.6.36 (specifically commit d57e34fd ("perf: Simplify the
ring-buffer logic: make perf_buffer_alloc() do everything needed"),
the perf_buffer_init_code() has been mis-setting the buffer watermark
if perf_event_attr.wakeup_events has a non-zero value.

This is because perf_event_attr.wakeup_events is a union with
perf_event_attr.wakeup_watermark.

This commit re-enables the check for perf_event_attr.watermark being
set before continuing with setting a non-default watermark.

This bug is most noticable when you are trying to use PERF_IOC_REFRESH
with a value larger than one and perf_event_attr.wakeup_events is set to
one.  In this case the buffer watermark will be set to 1 and you will
get extraneous POLL_IN overflows rather than POLL_HUP as expected.

[ avoid using attr.wakeup_events when attr.watermark is set ]

Signed-off-by: Vince Weaver <vweaver1@eecs.utk.edu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.00.1106011506390.5384@cl320.eecs.utk.edu


Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 0f933625

kernel/events/core.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -3569,8 +3569,10 @@ static int perf_mmap(struct file file, struct vm_area_struct vma)
		if (vma->vm_flags & VM_WRITE)
		flags \|= RING_BUFFER_WRITABLE;

		rb = rb_alloc(nr_pages, event->attr.wakeup_watermark,
		rb = rb_alloc(nr_pages,
		event->attr.watermark ? event->attr.wakeup_watermark : 0,
		event->cpu, flags);

		if (!rb) {
		ret = -ENOMEM;
		goto unlock;

kernel/events/ring_buffer.c

+9 −7

Original line number	Diff line number	Diff line
		@@ -199,15 +199,17 @@ void perf_output_end(struct perf_output_handle *handle)
		struct perf_event *event = handle->event;
		struct ring_buffer *rb = handle->rb;

		if (handle->sample && !event->attr.watermark) {
		int wakeup_events = event->attr.wakeup_events;

		if (handle->sample && wakeup_events) {
		if (wakeup_events) {
		int events = local_inc_return(&rb->events);
		if (events >= wakeup_events) {
		local_sub(wakeup_events, &rb->events);
		local_inc(&rb->wakeup);
		}
		}
		}

		perf_output_put_handle(handle);
		rcu_read_unlock();

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