linux-kernel - [patch] perf_events: fix perf buffer watermark setting

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Date:	Wed, 1 Jun 2011 15:15:36 -0400 (EDT)
From:	Vince Weaver <vweaver1@...s.utk.edu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	acme@...hat.com
Subject: [patch] perf_events: fix perf buffer watermark setting

Since 2.6.36 (specifically commit d57e34fdd60be7ffd0b1d86bfa1a553df86b7172)
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.

Signed-off-by: Vince Weaver <vweaver1@...s.utk.edu>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index d863b3c..b949661 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3787,7 +3787,8 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	if (vma->vm_flags & VM_WRITE)
 		flags |= PERF_BUFFER_WRITABLE;

-	buffer = perf_buffer_alloc(nr_pages, event->attr.wakeup_watermark,
+	buffer = perf_buffer_alloc(nr_pages, 
+				   event->attr.watermark?event->attr.wakeup_watermark:0,
 				   event->cpu, flags);
 	if (!buffer) {
 		ret = -ENOMEM;
--
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