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.rutl...@arm.com>
> Cc: Alexander Shishkin <alexander.shish...@linux.intel.com>
> Cc: Arnaldo Carvalho de Melo <a...@kernel.org>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: linux-kernel@vger.kernel.org
> ---
> 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.rutl...@arm.com
>
> 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
