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

From: Rob Herring <robh@kernel.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Raphael Gault <raphael.gault@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Ian Rogers <irogers@google.com>,
	Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Subject: Re: [PATCH v3 05/10] libperf: Add libperf_evsel__mmap()
Date: Tue, 22 Sep 2020 09:28:44 -0600	[thread overview]
Message-ID: <CAL_JsqJWGDwDa8bayjJ6L4hpwhekUKSngiynbyRw2WRWZeA8YA@mail.gmail.com> (raw)
In-Reply-To: <20200918143331.GF2626435@krava>

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