linux-kernel - [PATCH] perf stat: Do not delay the workload with --delay

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Date:   Mon, 12 Dec 2022 15:08:20 -0800
From:   Namhyung Kim <namhyung@...nel.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-perf-users@...r.kernel.org,
        Sumanth Korikkar <sumanthk@...ux.ibm.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Kevin Nomura <nomurak@...gle.com>
Subject: [PATCH] perf stat: Do not delay the workload with --delay

The -D/--delay option is to delay the measure after the program starts.
But the current code goes to sleep before starting the program so the
program is delayed too.  This is not the intention, let's fix it.

Before:

  $ time sudo ./perf stat -a -e cycles -D 3000 sleep 4
  Events disabled
  Events enabled

   Performance counter stats for 'system wide':

       4,326,949,337      cycles

         4.007494118 seconds time elapsed

  real	0m7.474s
  user	0m0.356s
  sys	0m0.120s

It ran the workload for 4 seconds and gave the 3 second delay.  So it
should skip the first 3 second and measure the last 1 second only.  But
as you can see, it delays 3 seconds and ran the workload after that for
4 seconds.  So the total time (real) was 7 seconds.

After:

  $ time sudo ./perf stat -a -e cycles -D 3000 sleep 4
  Events disabled
  Events enabled

   Performance counter stats for 'system wide':

       1,063,551,013      cycles

         1.002769510 seconds time elapsed

  real	0m4.484s
  user	0m0.385s
  sys	0m0.086s

The bug was introduced when it changed enablement of system-wide events
with a command line workload.  But it should've considered the initial
delay case.  The code was reworked since then (in bb8bc52e7578) so I'm
afraid it won't be applied cleanly.

Fixes: d0a0a511493d ("perf stat: Fix forked applications enablement of counters")
Cc: Sumanth Korikkar <sumanthk@...ux.ibm.com>
Cc: Thomas Richter <tmricht@...ux.ibm.com>
Reported-by: Kevin Nomura <nomurak@...gle.com>
Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/builtin-stat.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index d040fbcdcc5a..b39bf785a16e 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -540,26 +540,14 @@ static int enable_counters(void)
 			return err;
 	}
 
-	if (stat_config.initial_delay < 0) {
-		pr_info(EVLIST_DISABLED_MSG);
-		return 0;
-	}
-
-	if (stat_config.initial_delay > 0) {
-		pr_info(EVLIST_DISABLED_MSG);
-		usleep(stat_config.initial_delay * USEC_PER_MSEC);
-	}
-
 	/*
 	 * We need to enable counters only if:
 	 * - we don't have tracee (attaching to task or cpu)
 	 * - we have initial delay configured
 	 */
-	if (!target__none(&target) || stat_config.initial_delay) {
+	if (!target__none(&target)) {
 		if (!all_counters_use_bpf)
 			evlist__enable(evsel_list);
-		if (stat_config.initial_delay > 0)
-			pr_info(EVLIST_ENABLED_MSG);
 	}
 	return 0;
 }
@@ -930,14 +918,27 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx)
 			return err;
 	}
 
-	err = enable_counters();
-	if (err)
-		return -1;
+	if (stat_config.initial_delay) {
+		pr_info(EVLIST_DISABLED_MSG);
+	} else {
+		err = enable_counters();
+		if (err)
+			return -1;
+	}
 
 	/* Exec the command, if any */
 	if (forks)
 		evlist__start_workload(evsel_list);
 
+	if (stat_config.initial_delay > 0) {
+		usleep(stat_config.initial_delay * USEC_PER_MSEC);
+		err = enable_counters();
+		if (err)
+			return -1;
+
+		pr_info(EVLIST_ENABLED_MSG);
+	}
+
 	t0 = rdclock();
 	clock_gettime(CLOCK_MONOTONIC, &ref_time);
 
-- 
2.39.0.rc1.256.g54fd8350bd-goog