Re: [PATCH v9 4/4] tools/perf: Allow inherit + PERF_SAMPLE_READ when opening events

[Ben Gainey <Ben.Gainey@arm.com>]

On Wed, 2024-07-31 at 11:17 -0700, Namhyung Kim wrote:
> Hello,
> 
> On Tue, Jul 30, 2024 at 09:44:17AM +0100, Ben Gainey wrote:
> > The "perf record" tool will now default to this new mode if the
> > user
> > specifies a sampling group when not in system-wide mode, and when
> > "--no-inherit" is not specified.
> > 
> > This change updates evsel to allow the combination of inherit
> > and PERF_SAMPLE_READ.
> > 
> > A fallback is implemented for kernel versions where this feature is
> > not
> > supported.
> > 
> > Signed-off-by: Ben Gainey <ben.gainey@arm.com>
> > ---
> > 

snip


> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index bc603193c477..ceb09b6a8c2f 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -1171,7 +1171,15 @@ void evsel__config(struct evsel *evsel,
> > struct record_opts *opts,
> >   */
> >   if (leader->core.nr_members > 1) {
> >   attr->read_format |= PERF_FORMAT_GROUP;
> > - attr->inherit = 0;
> > + }
> > +
> > + /*
> > + * Inherit + SAMPLE_READ requires SAMPLE_TID in the read_format
> > + */
> > + if (attr->inherit) {
> > + evsel__set_sample_bit(evsel, TID);
> > + evsel->core.attr.read_format |=
> > + PERF_FORMAT_ID;
> >   }
> 
> Also I think we should reset the inherit bit for system-wide events.
> 
>   $ perf record -a --synth=no true
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 0.042 MB perf.data (51 samples) ]
>   
>   $ perf evlist -v | tr ',' '\n' | grep inherit
>    inherit: 1
>    inherit: 1
> 
> Maybe something like this:
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index bc603193c477..9423cd65c3c4 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -1149,7 +1149,7 @@ void evsel__config(struct evsel *evsel, struct
> record_opts *opts,
>         bool per_cpu = opts->target.default_per_cpu && !opts-
> >target.per_thread;
>  
>         attr->sample_id_all = perf_missing_features.sample_id_all ? 0
> : 1;
> -       attr->inherit       = !opts->no_inherit;
> +       attr->inherit       = target__has_cpu(&opts->target) ? 0 :
> !opts->no_inherit;
>         attr->write_backward = opts->overwrite ? 1 : 0;
>         attr->read_format   = PERF_FORMAT_LOST;
>  
> 
> Thanks,
> Namhyung
> 

Done, though this looks benign; the current tools appear to do this,
and there are a couple of files in tests/attr that assumed inherit=1
for system wide mode that I've had to update.


   # perf --version
   perf version 6.10-1
   # perf record -vvv -a --synth=no true
   ...
   ------------------------------------------------------------
   perf_event_attr:
     type                             0 (PERF_TYPE_HARDWARE)
     size                             136
     config                           0 (PERF_COUNT_HW_CPU_CYCLES)
     { sample_period, sample_freq }   4000
     sample_type                      IP|TID|TIME|CPU|PERIOD|IDENTIFIER
     read_format                      ID|LOST
     disabled                         1
     inherit                          1
     freq                             1
     precise_ip                       3
     sample_id_all                    1
   ------------------------------------------------------------

Testing your fix with -a, -C <n> as well as with application profiling
appears to do the right thing.


Regards
Ben


> 
> >   }
> >  
> > @@ -2020,6 +2028,8 @@ static int __evsel__prepare_open(struct evsel
> > *evsel, struct perf_cpu_map *cpus,
> >  
> >  static void evsel__disable_missing_features(struct evsel *evsel)
> >  {
> > + if (perf_missing_features.inherit_sample_read)
> > + evsel->core.attr.inherit = 0;
> >   if (perf_missing_features.branch_counters)
> >   evsel->core.attr.branch_sample_type &=
> > ~PERF_SAMPLE_BRANCH_COUNTERS;
> >   if (perf_missing_features.read_lost)
> > @@ -2075,7 +2085,12 @@ bool evsel__detect_missing_features(struct
> > evsel *evsel)
> >   * Must probe features in the order they were added to the
> >   * perf_event_attr interface.
> >   */
> > - if (!perf_missing_features.branch_counters &&
> > + if (!perf_missing_features.inherit_sample_read &&
> > +     evsel->core.attr.inherit && (evsel->core.attr.sample_type &
> > PERF_SAMPLE_READ)) {
> > + perf_missing_features.inherit_sample_read = true;
> > + pr_debug2("Using PERF_SAMPLE_READ / :S modifier is not compatible
> > with inherit, falling back to no-inherit.\n");
> > + return true;
> > + } else if (!perf_missing_features.branch_counters &&
> >       (evsel->core.attr.branch_sample_type &
> > PERF_SAMPLE_BRANCH_COUNTERS)) {
> >   perf_missing_features.branch_counters = true;
> >   pr_debug2("switching off branch counters support\n");
> > diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> > index 80b5f6dd868e..bb0c91c23679 100644
> > --- a/tools/perf/util/evsel.h
> > +++ b/tools/perf/util/evsel.h
> > @@ -206,6 +206,7 @@ struct perf_missing_features {
> >   bool weight_struct;
> >   bool read_lost;
> >   bool branch_counters;
> > + bool inherit_sample_read;
> >  };
> >  
> >  extern struct perf_missing_features perf_missing_features;
> > -- 
> > 2.45.2
> >