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

Rob Herring
2020-09-22T15:28:59Z

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.

Rob