linux-kernel - Re: [PATCH] libperf: Add perf_evsel_

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Message-ID: <Ztn/mRDqPSpmliWq@ghost>
Date: Thu, 5 Sep 2024 11:59:37 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] libperf: Add perf_evsel__id() function

On Wed, Sep 04, 2024 at 09:22:33PM -0700, Namhyung Kim wrote:
> Hello,
> 
> On Fri, Aug 23, 2024 at 11:58:38AM -0700, Charlie Jenkins wrote:
> > Introduce perf_evsel__id() to collect the id of an evsel. It is not
> > currently possible when using libperf to determine the id of an evsel.
> > This will allow applications to link the id returned by PERF_SAMPLE_ID
> > to the event being sampled.
> 
> I'm not sure what's your use case.  We have evlist__id2evsel() to
> convert sample-ID to evsel already.  It'd read the IDs from perf data
> file.

That function is unfortunately not exposed to be used by applications.
This patch is for libperf, not for the util internals of userspace perf.
Since I am using libperf (and not the python wrapper) I am collecting
data directly from the mmap buffer and not from the perf data file. The
mmap buffer only contains the id of the sample, and no way of
associating this id with the evsel that is backing the id using only
libperf API functions. A libperf function that provides a pointer to the
associated evsel could be another alternative to this. However an API
that returns a pointer to the evsel is not very useful as libperf
applications cannot read members of perf structs, as the layout of perf
structs are not part of the API.

> 
> > 
> > Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> > ---
> >  tools/lib/perf/Documentation/libperf.txt |  2 ++
> >  tools/lib/perf/evsel.c                   | 10 ++++++++++
> >  tools/lib/perf/include/perf/evsel.h      |  1 +
> >  3 files changed, 13 insertions(+)
> > 
> > diff --git a/tools/lib/perf/Documentation/libperf.txt b/tools/lib/perf/Documentation/libperf.txt
> > index fcfb9499ef9c..69c1d7efb659 100644
> > --- a/tools/lib/perf/Documentation/libperf.txt
> > +++ b/tools/lib/perf/Documentation/libperf.txt
> > @@ -94,6 +94,8 @@ SYNOPSIS
> >    void perf_evlist__enable(struct perf_evlist *evlist);
> >    void perf_evlist__disable(struct perf_evlist *evlist);
> >  
> > +  void perf_evsel__id(struct perf_evsel *evsel, u64 *id);
> > +
> >    #define perf_evlist__for_each_evsel(evlist, pos)
> >  
> >    void perf_evlist__set_maps(struct perf_evlist *evlist,
> > diff --git a/tools/lib/perf/evsel.c b/tools/lib/perf/evsel.c
> > index c07160953224..765b17045342 100644
> > --- a/tools/lib/perf/evsel.c
> > +++ b/tools/lib/perf/evsel.c
> > @@ -484,6 +484,16 @@ int perf_evsel__disable(struct perf_evsel *evsel)
> >  	return err;
> >  }
> >  
> > +int perf_evsel__id(struct perf_evsel *evsel, __u64 *id)
> > +{
> > +	int i;
> > +	int err = 0;
> > +
> > +	for (i = 0; i < xyarray__max_x(evsel->fd) && !err; i++)
> > +		err = perf_evsel__run_ioctl(evsel, PERF_EVENT_IOC_ID, (unsigned long)id, i);
> 
> IIUC this would update the place pointed by 'id' for every fd then you
> will get the last ID only.

Yes I have a misunderstanding with how this is supposed to be written.
Can there be multiple ids associated with a given evsel? When parsing
the data in the mmap buffer, I want to be able to associate a sample id
with the event encoding. I suppose an API to get the evsel from an id,
and then require libperf applications to search some data structure to
get the associated event with the evsel is reasonable.

- Charlie

> 
> Thanks,
> Namhyung
> 
> > +	return err;
> > +}
> > +
> >  int perf_evsel__apply_filter(struct perf_evsel *evsel, const char *filter)
> >  {
> >  	int err = 0, i;
> > diff --git a/tools/lib/perf/include/perf/evsel.h b/tools/lib/perf/include/perf/evsel.h
> > index 6f92204075c2..1457e5a46b28 100644
> > --- a/tools/lib/perf/include/perf/evsel.h
> > +++ b/tools/lib/perf/include/perf/evsel.h
> > @@ -41,6 +41,7 @@ LIBPERF_API int perf_evsel__enable_cpu(struct perf_evsel *evsel, int cpu_map_idx
> >  LIBPERF_API int perf_evsel__enable_thread(struct perf_evsel *evsel, int thread);
> >  LIBPERF_API int perf_evsel__disable(struct perf_evsel *evsel);
> >  LIBPERF_API int perf_evsel__disable_cpu(struct perf_evsel *evsel, int cpu_map_idx);
> > +LIBPERF_API int perf_evsel__id(struct perf_evsel *evsel, __u64 *id);
> >  LIBPERF_API struct perf_cpu_map *perf_evsel__cpus(struct perf_evsel *evsel);
> >  LIBPERF_API struct perf_thread_map *perf_evsel__threads(struct perf_evsel *evsel);
> >  LIBPERF_API struct perf_event_attr *perf_evsel__attr(struct perf_evsel *evsel);
> > 
> > ---
> > base-commit: 47ac09b91befbb6a235ab620c32af719f8208399
> > change-id: 20240822-perf_evsel_get_id-f7e11f15504b
> > -- 
> > - Charlie
> >