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Message-Id: <20101119215639.150248565@clark.site>
Date: Fri, 19 Nov 2010 13:55:26 -0800
From: Greg KH <gregkh@...e.de>
To: linux-kernel@...r.kernel.org, stable@...nel.org
Cc: stable-review@...nel.org, torvalds@...ux-foundation.org,
akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
Stephane Eranian <eranian@...gle.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...e.hu>
Subject: [05/82] perf_events: Fix bogus context time tracking
2.6.35-stable review patch. If anyone has any objections, please let us know.
------------------
From: Stephane Eranian <eranian@...gle.com>
commit c530ccd9a1864a44a7ff35826681229ce9f2357a upstream.
You can only call update_context_time() when the context
is active, i.e., the thread it is attached to is still running.
However, perf_event_read() can be called even when the context
is inactive, e.g., user read() the counters. The call to
update_context_time() must be conditioned on the status of
the context, otherwise, bogus time_enabled, time_running may
be returned. Here is an example on AMD64. The task program
is an example from libpfm4. The -p prints deltas every 1s.
$ task -p -e cpu_clk_unhalted sleep 5
2,266,610 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
5,242,358,071 cpu_clk_unhalted (99.95% scaling, ena=5,000,359,984, run=2,319,270)
Whereas if you don't read deltas, e.g., no call to perf_event_read() until
the process terminates:
$ task -e cpu_clk_unhalted sleep 5
2,497,783 cpu_clk_unhalted (0.00% scaling, ena=2,376,899, run=2,376,899)
Notice that time_enable, time_running are bogus in the first example
causing bogus scaling.
This patch fixes the problem, by conditionally calling update_context_time()
in perf_event_read().
Signed-off-by: Stephane Eranian <eranian@...gle.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
LKML-Reference: <4cb856dc.51edd80a.5ae0.38fb@...google.com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
---
kernel/perf_event.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1757,7 +1757,13 @@ static u64 perf_event_read(struct perf_e
unsigned long flags;
raw_spin_lock_irqsave(&ctx->lock, flags);
- update_context_time(ctx);
+ /*
+ * may read while context is not active
+ * (e.g., thread is blocked), in that case
+ * we cannot update context time
+ */
+ if (ctx->is_active)
+ update_context_time(ctx);
update_event_times(event);
raw_spin_unlock_irqrestore(&ctx->lock, flags);
}
--
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