[tip:perf/urgent] perf stat: Avoid skew when reading events

[tip-bot for Mark Rutland]

Commit-ID:  3df33eff2ba96be4f1535db4f672013d756dc9b1
Gitweb:     http://git.kernel.org/tip/3df33eff2ba96be4f1535db4f672013d756dc9b1
Author:     Mark Rutland <mark.rutland@xxxxxxx>
AuthorDate: Tue, 9 Aug 2016 14:04:29 +0100
Committer:  Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
CommitDate: Tue, 9 Aug 2016 10:48:32 -0300

perf stat: Avoid skew when reading events

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 removes the event closing logic from the
read_counters() helper, explicitly closes the events using
perf_evlist__close(), which also aids legibility.

Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Tested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1470747869-3567-1-git-send-email-mark.rutland@xxxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
 tools/perf/builtin-stat.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 0c16d20..3c7452b 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 +341,6 @@ 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));
-		}
 	}
 }
 
@@ -353,7 +348,7 @@ 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 +375,17 @@ 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 +663,20 @@ 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();
+	perf_evlist__close(evsel_list);
 
 	return WEXITSTATUS(status);
 }