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

[Leo Yan <leo.yan@linaro.org>]

On Mon, Dec 12, 2022 at 03:08:20PM -0800, Namhyung Kim wrote:
> 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@linux.ibm.com>
> Cc: Thomas Richter <tmricht@linux.ibm.com>
> Reported-by: Kevin Nomura <nomurak@google.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Reviewed-by: Leo Yan <leo.yan@linaro.org>