Messages in this thread Patch in this message | | | From | Stephane Eranian <> | Date | Fri, 15 Oct 2010 15:26:01 +0200 | Subject | [PATCH] perf_events: fix bogus context time tracking |
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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>
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diff --git a/kernel/perf_event.c b/kernel/perf_event.c index b6fd9ec..4dae345 100644 --- a/kernel/perf_event.c +++ b/kernel/perf_event.c @@ -1775,7 +1775,13 @@ 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); }
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