On Fri, 2010-10-15 at 15:26 +0200, Stephane Eranian wrote: > 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 <eran...@google.com>
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