linux-kernel - [PATCH 03/21] perf x86: Update event count properly for read syscall

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Date:	Wed, 25 Sep 2013 14:50:29 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [PATCH 03/21] perf x86: Update event count properly for read syscall

Within the read syscall we update the event count via
pmu->read callback if the event is in ACTIVE state.

This triggers event update (x86_perf_event_update) for
x86 pmu and leads to wrong event count in case the event
hasn't been started yet -> the event base is not adjusted
to new cpu and new count computation is based on previous
CPU prev_count.

Fixing this to update the counter only if it has been
started, so the event base is properly adjusted.

Signed-off-by: Jiri Olsa <jolsa@...hat.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Corey Ashford <cjashfor@...ux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Ingo Molnar <mingo@...e.hu>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 arch/x86/kernel/cpu/perf_event.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 8355c84..e89c773 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1570,7 +1570,11 @@ early_initcall(init_hw_perf_events);
 
 static inline void x86_pmu_read(struct perf_event *event)
 {
-	x86_perf_event_update(event);
+	struct hw_perf_event *hwc = &event->hw;
+
+	/* Update only if the event has already been started. */
+	if (!(hwc->state & PERF_HES_ARCH))
+		x86_perf_event_update(event);
 }
 
 /*
-- 
1.7.11.7

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