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authorStephane Eranian <eranian@google.com>2010-10-15 15:26:01 +0200
committerGreg Kroah-Hartman <gregkh@suse.de>2010-11-22 11:03:00 -0800
commit603cb849c8c657e08cead0bb6c93e1fa88025c78 (patch)
treea828e4015d51ee06ed83a5633e6bb9da56fb8829 /kernel
parentec0c107232ded1f4485eb55248fb572635c4b370 (diff)
perf_events: Fix bogus context time tracking
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@google.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> LKML-Reference: <4cb856dc.51edd80a.5ae0.38fb@mx.google.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/perf_event.c8
1 files changed, 7 insertions, 1 deletions
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index b98bed3d8182..03bb8974bc90 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1773,7 +1773,13 @@ static u64 perf_event_read(struct perf_event *event)
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);
}