Re: [PATCH v2] perf: disable PERF_RECORD_MMAP2 support

[David Ahern]

On 10/17/13 9:57 AM, Stephane Eranian wrote:
> On Thu, Oct 17, 2013 at 5:54 PM, David Ahern <dsahern@xxxxxxxxx> wrote:
> > On 10/17/13 8:28 AM, Stephane Eranian wrote:
> > > For now, we disable the extended MMAP record support (MMAP2).
> > > We have identified cases where it would not report the correct
> > > mapping information, clone(VM_CLONE) but with separate pids.
> > > We will revisit the support once we find a solution for this case.
> > >
> > > The patch changes the kernel to return EINVAL if attr->mmap2
> > > is set. The patch also modifies the perf tool to use regular
> > > PERF_RECORD_MMAP
> > > for synthetic events and it also prevents the tool from requesting
> > > attr->mmap2
> > > mode because the kernel would reject it.
> > >
> > > The support will be revisited once the kenrel interface is updated.
> >
> >
> > Why not disable mmap2 as well:
> >
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index 0ce9febf1ba0..289f34dbe970 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -678,7 +678,7 @@ void perf_evsel__config(struct perf_evsel *evsel,
> >          attr->sample_type   |= PERF_SAMPLE_WEIGHT;
> >
> >      attr->mmap  = track;
> > -   attr->mmap2 = track && !perf_missing_features.mmap2;
> > +   /* attr->mmap2 = track && !perf_missing_features.mmap2; */
> >      attr->comm  = track;
> It is disabled automatically later on. I checked that. Wanted to minimize
> the changes.

Understood that the fallback kicks in. Why fail the perf_event_open 
every single time when we know it is not going to succeed?

David
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