linux-kernel - Re: [PATCH] perf_events: fix bogus context time tracking

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Date:	Fri, 15 Oct 2010 15:54:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	eranian@...gle.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com,
	robert.richter@....com
Subject: Re: [PATCH] perf_events: fix bogus context time tracking

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 <eranian@...gle.com>

Thanks, tagged it for -stable as well.
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