[tip:perf/core] perf/x86-ibs: Extend hw period that triggers overflow

[tip-bot for Robert Richter]

Commit-ID:  7caaf4d8241feecafb87919402b0a6dbb1b71d9e
Gitweb:     http://git.kernel.org/tip/7caaf4d8241feecafb87919402b0a6dbb1b71d9e
Author:     Robert Richter <robert.richter@xxxxxxx>
AuthorDate: Mon, 2 Apr 2012 20:19:15 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 9 May 2012 15:23:15 +0200

perf/x86-ibs: Extend hw period that triggers overflow

If the last hw period is too short we might hit the irq handler which
biases the results. Thus try to have a max last period that triggers
the sw overflow.

Signed-off-by: Robert Richter <robert.richter@xxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1333390758-10893-10-git-send-email-robert.richter@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/perf_event_amd_ibs.c |   15 +++++++++++++--
 1 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_amd_ibs.c b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
index 3e32908..cb51a3e 100644
--- a/arch/x86/kernel/cpu/perf_event_amd_ibs.c
+++ b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
@@ -85,8 +85,19 @@ perf_event_set_period(struct hw_perf_event *hwc, u64 min, u64 max, u64 *hw_perio
 		overflow = 1;
 	}
 
-	if (left > max)
-		left = max;
+	/*
+	 * If the hw period that triggers the sw overflow is too short
+	 * we might hit the irq handler. This biases the results.
+	 * Thus we shorten the next-to-last period and set the last
+	 * period to the max period.
+	 */
+	if (left > max) {
+		left -= max;
+		if (left > max)
+			left = max;
+		else if (left < min)
+			left = min;
+	}
 
 	*hw_period = (u64)left;
 
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