linux-kernel - 'perf stat --repeat N' oddity/regression

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Date:	Mon, 25 Jan 2016 19:22:06 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: 'perf stat --repeat N' oddity/regression


So I just noticed this 'perf stat --repeat' oddity:

triton:~/tip> perf stat --repeat 3 -a -e instructions taskset 1 perf bench sched pipe -l 1000000
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

     Total time: 2.068 [sec]

       2.068208 usecs/op
         483510 ops/sec
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

     Total time: 2.097 [sec]

       2.097126 usecs/op
         476843 ops/sec
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

     Total time: 2.091 [sec]

       2.091716 usecs/op
         478076 ops/sec

 Performance counter stats for 'system wide' (3 runs):

    12,690,156,407      instructions                                                

       2.087612423 seconds time elapsed                                          ( +-  0.43% )

the stddev printout of 'instructions' is gone! This defeats the main purpose of 
--repeat.

The 'elapsed' time +- stddev/noise display (which is still present) is only part 
of the story.

I'm pretty sure we had the printout for all the measured fields a couple of months 
ago - does any of you know what happened to it?

Thanks,

	Ingo