Re: [PATCH v4 4/9] libperf: Add libperf_evsel__mmap()

[Rob Herring]

On Wed, Oct 14, 2020 at 6:05 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> On Thu, Oct 01, 2020 at 09:01:11AM -0500, Rob Herring wrote:
>
> SNIP
>
> >
> > +void *perf_evsel__mmap(struct perf_evsel *evsel, int pages)
> > +{
> > +     int ret;
> > +     struct perf_mmap *map;
> > +     struct perf_mmap_param mp = {
> > +             .prot = PROT_READ | PROT_WRITE,
> > +     };
> > +
> > +     if (FD(evsel, 0, 0) < 0)
> > +             return NULL;
> > +
> > +     mp.mask = (pages * page_size) - 1;
> > +
> > +     map = zalloc(sizeof(*map));
> > +     if (!map)
> > +             return NULL;
> > +
> > +     perf_mmap__init(map, NULL, false, NULL);
> > +
> > +     ret = perf_mmap__mmap(map, &mp, FD(evsel, 0, 0), 0);
>
> hum, so you map event for FD(0,0) but later in perf_evsel__read
> you allow to read any cpu/thread combination ending up reading
> data from FD(0,0) map:
>
>         int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread,
>                              struct perf_counts_values *count)
>         {
>                 size_t size = perf_evsel__read_size(evsel);
>
>                 memset(count, 0, sizeof(*count));
>
>                 if (FD(evsel, cpu, thread) < 0)
>                         return -EINVAL;
>
>                 if (evsel->mmap && !perf_mmap__read_self(evsel->mmap, count))
>                         return 0;
>
>
> I think we should either check cpu == 0, thread == 0, or make it
> general and store perf_evsel::mmap in xyarray as we do for fds

The mmapped read will actually fail and then we fallback here. My main
concern though is adding more overhead on a feature that's meant to be
low overhead (granted, it's not much). Maybe we could add checks on
the mmap that we've opened the event with pid == 0 and cpu == -1 (so
only 1 FD)?

Rob