Re: [PATCH] perf: fix counting when initial delay configured

[Namhyung Kim]

Hello,

On Thu, Feb 23, 2023 at 5:45 AM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> Em Thu, Feb 23, 2023 at 03:58:00PM +0800, Changbin Du escreveu:
> > When creating counters with initial delay configured, the enable_on_exec
> > field is not set. So we need to enable the counters later. The problem
> > is, when a workload is specified the target__none() is still true. So
> > we also need to check stat_config.initial_delay.
> >
> > Before this fix the event is not counted:
> > $ ./perf stat -e instructions -D 100 sleep 2
> > Events disabled
> > Events enabled
> >
> >  Performance counter stats for 'sleep 2':
> >
> >      <not counted>      instructions
> >
> >        1.901661124 seconds time elapsed
> >
> >        0.001602000 seconds user
> >        0.000000000 seconds sys
> >
> > After fix it works:
> > $ ./perf stat -e instructions -D 100 sleep 2
> > Events disabled
> > Events enabled
> >
> >  Performance counter stats for 'sleep 2':
> >
> >            404,214      instructions
> >
> >        1.901743475 seconds time elapsed
> >
> >        0.001617000 seconds user
> >        0.000000000 seconds sys
> >
> > Fixes: c587e77e100f ("perf stat: Do not delay the workload with --delay")
>
> Yeap, even the comment states that we need to enable when initial_delay
> is set :-)

Right, but the logic that checks the initial_delay is placed
out of the function.  Just checking the initial_delay value
can be confusing as it can have a negative value.

Maybe we can add an argument (bool force?) to the
enable_counters() function.

Thanks,
Namhyung


>
> I added the additional test output below.
>
> Namhyung, can you please ack it?
>
> - Arnaldo
>
> Committer testing:
>
> Before:
>
> Lets use stress-ng so that we have lots of samples using a CPU stressor
> and also intermingle the workload output with the messages about when
> the events get enabled (i.e. later on in the workload):
>
>   $ perf stat -e instructions -D 100 stress-ng -c 32 -t 1
>   Events disabled
>   stress-ng: info:  [38361] setting to a 1 second run per stressor
>   stress-ng: info:  [38361] dispatching hogs: 32 cpu
>   Events enabled
>   stress-ng: info:  [38361] successful run completed in 1.01s
>
>    Performance counter stats for 'stress-ng -c 32 -t 1':
>
>        <not counted>      instructions:u
>
>          0.916479141 seconds time elapsed
>
>         30.868003000 seconds user
>          0.049851000 seconds sys
>
>
>   Some events weren't counted. Try disabling the NMI watchdog:
>         echo 0 > /proc/sys/kernel/nmi_watchdog
>         perf stat ...
>         echo 1 > /proc/sys/kernel/nmi_watchdog
>   $
>
> After the fix:
>
>   $ perf stat -e instructions -D 100 stress-ng -c 32 -t 1
>   Events disabled
>   stress-ng: info:  [40429] setting to a 1 second run per stressor
>   stress-ng: info:  [40429] dispatching hogs: 32 cpu
>   Events enabled
>   stress-ng: info:  [40429] successful run completed in 1.01s
>
>    Performance counter stats for 'stress-ng -c 32 -t 1':
>
>         154117865145      instructions:u
>
>          0.920827644 seconds time elapsed
>
>         30.864753000 seconds user
>          0.073862000 seconds sys
>
>
>   $
>
> > Signed-off-by: Changbin Du <changbin.du@xxxxxxxxxx>
> > ---
> >  tools/perf/builtin-stat.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
> > index 9f3e4b257516..c71d85577de6 100644
> > --- a/tools/perf/builtin-stat.c
> > +++ b/tools/perf/builtin-stat.c
> > @@ -544,7 +544,7 @@ static int enable_counters(void)
> >        * - we don't have tracee (attaching to task or cpu)
> >        * - we have initial delay configured
> >        */
> > -     if (!target__none(&target)) {
> > +     if (!target__none(&target) || stat_config.initial_delay) {
> >               if (!all_counters_use_bpf)
> >                       evlist__enable(evsel_list);
> >       }
> > --
> > 2.25.1
> >
>
> --
>
> - Arnaldo