Re: [PATCH v3 05/10] libperf: Add libperf_evsel__mmap()

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Sep 22, 2020 at 09:28:44AM -0600, Rob Herring wrote:
> On Fri, Sep 18, 2020 at 8:33 AM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Fri, Sep 11, 2020 at 03:51:13PM -0600, Rob Herring wrote:
> > > In order to support usersapce access, an event must be mmapped. While
> > > there's already mmap support for evlist, the usecase is a bit different
> > > than the self monitoring with userspace access. So let's add a new
> > > perf_evsel__mmap() function to mmap an evsel. This allows implementing
> > > userspace access as a fastpath for perf_evsel__read().
> > >
> > > The mmapped address is returned by perf_evsel__mmap() primarily for
> > > users/tests to check if userspace access is enabled.
> > >
> > > Signed-off-by: Rob Herring <robh@kernel.org>
> > > ---
> > > v3:
> > >  - New patch split out from user access patch
> > > ---
> > >  tools/lib/perf/Documentation/libperf.txt |  1 +
> > >  tools/lib/perf/evsel.c                   | 31 ++++++++++++++++++++++++
> > >  tools/lib/perf/include/internal/evsel.h  |  2 ++
> > >  tools/lib/perf/include/perf/evsel.h      |  2 ++
> > >  tools/lib/perf/libperf.map               |  1 +
> > >  5 files changed, 37 insertions(+)
> > >
> > > diff --git a/tools/lib/perf/Documentation/libperf.txt b/tools/lib/perf/Documentation/libperf.txt
> > > index 0c74c30ed23a..0b4694ce42b9 100644
> > > --- a/tools/lib/perf/Documentation/libperf.txt
> > > +++ b/tools/lib/perf/Documentation/libperf.txt
> > > @@ -136,6 +136,7 @@ SYNOPSIS
> > >                         struct perf_thread_map *threads);
> > >    void perf_evsel__close(struct perf_evsel *evsel);
> > >    void perf_evsel__close_cpu(struct perf_evsel *evsel, int cpu);
> > > +  void *perf_evsel__mmap(struct perf_evsel *evsel, size_t length);
> > >    int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread,
> > >                         struct perf_counts_values *count);
> > >    int perf_evsel__enable(struct perf_evsel *evsel);
> > > diff --git a/tools/lib/perf/evsel.c b/tools/lib/perf/evsel.c
> > > index 4dc06289f4c7..99fa53dc0887 100644
> > > --- a/tools/lib/perf/evsel.c
> > > +++ b/tools/lib/perf/evsel.c
> > > @@ -11,10 +11,12 @@
> > >  #include <stdlib.h>
> > >  #include <internal/xyarray.h>
> > >  #include <internal/cpumap.h>
> > > +#include <internal/mmap.h>
> > >  #include <internal/threadmap.h>
> > >  #include <internal/lib.h>
> > >  #include <linux/string.h>
> > >  #include <sys/ioctl.h>
> > > +#include <sys/mman.h>
> > >
> > >  void perf_evsel__init(struct perf_evsel *evsel, struct perf_event_attr *attr)
> > >  {
> > > @@ -156,6 +158,35 @@ void perf_evsel__close_cpu(struct perf_evsel *evsel, int cpu)
> > >       perf_evsel__close_fd_cpu(evsel, cpu);
> > >  }
> > >
> > > +void *perf_evsel__mmap(struct perf_evsel *evsel, size_t length)
> > > +{
> >
> > could you rather use 'int pages' insted of length as we
> > do in perf_evlist__mmap, so we keep the same interface
> >
> >   len = (pages + 1) * page_size
> 
> Sure, but what's the reason it is pages minus 1? It seems a bit odd to
> pass in 0 here to get 1 page which would be the typical usage.

the user mmap page is extra, so when you map 2 pages you actualy map 3,
but we want the size of what's available for storing events in the API

jirka