Re: [PATCH] perf-stat: avoid skew when reading events

[Arnaldo Carvalho de Melo]

Em Mon, Aug 08, 2016 at 12:16:23PM +0100, Mark Rutland escreveu:
> When we don't have a tracee (i.e. we're attaching to a task or CPU),
> counters can still be running after our workload finishes, and can still
> be running as we read their values. As we read events one-by-one, there
> can be arbitrary skew between values of events, even within a group.
> This means that ratios within an event group are not reliable.
> 
> This skew can be seen if measuring a group of identical events, e.g:
> 
>  # perf stat -a -C0 -e '{cycles,cycles}' sleep 1
> 
> To avoid this, we must stop groups from counting before we read the
> values of any constituent events. This patch adds and makes use of a new
> disable_counters() helper, which disables group leaders (and thus each
> group as a whole). This mirrors the use of enable_counters() for
> starting event groups in the absence of a tracee.
> 
> Closing a group leader splits the group, and without a disabled group
> leader the newly split events will begin counting. Thus to ensure counts
> are reliable we must defer closing group leaders until all counts have
> been read. To do so this patch splits the read_counters() helper into
> read_counters() and close_counters(), which also aids legibility.

Looks sane, just one nit, now that you close all counters in an evlist
in a loop, please just use perf_evlist__close(evsel_list), should be
equivalent to this new close_counters() routine.

- Arnaldo
 
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  tools/perf/builtin-stat.c | 39 ++++++++++++++++++++++++++++++++-------
>  1 file changed, 32 insertions(+), 7 deletions(-)
> 
> Note: there is also a kernel-side issue which adds skew to group counts, which
> is addressed by [1]. In the absence of [1], this patch minimises skew, but does
> not remove it entirely.
> 
> Mark.
> 
> [1] http://lkml.kernel.org/r/1469553141-28314-1-git-send-email-mark.rutland@xxxxxxx
> 
> diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> index 0c16d20..e8464b3 100644
> --- a/tools/perf/builtin-stat.c
> +++ b/tools/perf/builtin-stat.c
> @@ -331,7 +331,7 @@ static int read_counter(struct perf_evsel *counter)
>  	return 0;
>  }
>  
> -static void read_counters(bool close_counters)
> +static void read_counters(void)
>  {
>  	struct perf_evsel *counter;
>  
> @@ -341,11 +341,16 @@ static void read_counters(bool close_counters)
>  
>  		if (perf_stat_process_counter(&stat_config, counter))
>  			pr_warning("failed to process counter %s\n", counter->name);
> +	}
> +}
>  
> -		if (close_counters) {
> -			perf_evsel__close_fd(counter, perf_evsel__nr_cpus(counter),
> -					     thread_map__nr(evsel_list->threads));
> -		}
> +static void close_counters(void)
> +{
> +	struct perf_evsel *counter;
> +
> +	evlist__for_each(evsel_list, counter) {
> +		perf_evsel__close_fd(counter, perf_evsel__nr_cpus(counter),
> +				     thread_map__nr(evsel_list->threads));
>  	}
>  }
>  
> @@ -353,7 +358,7 @@ static void process_interval(void)
>  {
>  	struct timespec ts, rs;
>  
> -	read_counters(false);
> +	read_counters();
>  
>  	clock_gettime(CLOCK_MONOTONIC, &ts);
>  	diff_timespec(&rs, &ts, &ref_time);
> @@ -380,6 +385,17 @@ static void enable_counters(void)
>  		perf_evlist__enable(evsel_list);
>  }
>  
> +static void disable_counters(void)
> +{
> +	/*
> +	 * If we don't have tracee (attaching to task or cpu), counters may
> +	 * still be running. To get accurate group ratios, we must stop groups
> +	 * from counting before reading their constituent counters.
> +	 */
> +	if (!target__none(&target))
> +		perf_evlist__disable(evsel_list);
> +}
> +
>  static volatile int workload_exec_errno;
>  
>  /*
> @@ -657,11 +673,20 @@ try_again:
>  		}
>  	}
>  
> +	disable_counters();
> +
>  	t1 = rdclock();
>  
>  	update_stats(&walltime_nsecs_stats, t1 - t0);
>  
> -	read_counters(true);
> +	/*
> +	 * Closing a group leader splits the group, and as we only disable
> +	 * group leaders, results in remaining events becoming enabled. To
> +	 * avoid arbitrary skew, we must read all counters before closing any
> +	 * group leaders.
> +	 */
> +	read_counters();
> +	close_counters();
>  
>  	return WEXITSTATUS(status);
>  }
> -- 
> 1.9.1