linux-kernel - [tip:perf/core] perf/x86-ibs: Trigger overflow if remaining period is too small

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Date:	Wed, 9 May 2012 07:35:42 -0700
From:	tip-bot for Robert Richter <robert.richter@....com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	robert.richter@....com, a.p.zijlstra@...llo.nl, tglx@...utronix.de
Subject: [tip:perf/core] perf/x86-ibs:
  Trigger overflow if remaining period is too small

Commit-ID:  fc006cf7cc7471e1bdf34e40111971e03622af6c
Gitweb:     http://git.kernel.org/tip/fc006cf7cc7471e1bdf34e40111971e03622af6c
Author:     Robert Richter <robert.richter@....com>
AuthorDate: Mon, 2 Apr 2012 20:19:14 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 9 May 2012 15:23:15 +0200

perf/x86-ibs: Trigger overflow if remaining period is too small

There are cases where the remaining period is smaller than the minimal
possible value. In this case the counter is restarted with the minimal
period. This is of no use as the interrupt handler will trigger
immediately again and most likely hits itself. This biases the
results.

So, if the remaining period is within the min range, we better do not
restart the counter and instead trigger the overflow.

Signed-off-by: Robert Richter <robert.richter@....com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Link: http://lkml.kernel.org/r/1333390758-10893-9-git-send-email-robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/cpu/perf_event_amd_ibs.c |    5 +----
 1 files changed, 1 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_amd_ibs.c b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
index 29a1bff..3e32908 100644
--- a/arch/x86/kernel/cpu/perf_event_amd_ibs.c
+++ b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
@@ -78,16 +78,13 @@ perf_event_set_period(struct hw_perf_event *hwc, u64 min, u64 max, u64 *hw_perio
 		overflow = 1;
 	}
 
-	if (unlikely(left <= 0)) {
+	if (unlikely(left < (s64)min)) {
 		left += period;
 		local64_set(&hwc->period_left, left);
 		hwc->last_period = period;
 		overflow = 1;
 	}
 
-	if (unlikely(left < min))
-		left = min;
-
 	if (left > max)
 		left = max;
 
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