Re: [PATCH] perf stat: force error in fallback on :k events

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Apr 14, 2020 at 07:38:17AM -0700, Ian Rogers wrote:
> On Tue, Apr 14, 2020 at 6:02 AM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Mon, Apr 13, 2020 at 04:55:15PM -0700, Ian Rogers wrote:
> > > From: Stephane Eranian <eranian@google.com>
> > >
> > > When it is not possible for a non-privilege perf command
> > > to monitor at the kernel level (:k), the fallback code forces
> > > a :u. That works if the event was previously monitoring both levels.
> > > But if the event was already constrained to kernel only, then it does
> > > not make sense to restrict it to user only.
> > > Given the code works by exclusion, a kernel only event would have:
> > > attr->exclude_user = 1
> > > The fallback code would add:
> > > attr->exclude_kernel = 1;
> > >
> > > In the end the end would not monitor in either the user level or kernel
> > > level. In other words, it would count nothing.
> > >
> > > An event programmed to monitor kernel only cannot be switched to user only
> > > without seriously warning the user.
> > >
> > > This patch forces an error in this case to make it clear the request
> > > cannot really be satisfied.
> > >
> > > Signed-off-by: Stephane Eranian <eranian@google.com>
> > > Reviewed-by: Ian Rogers <irogers@google.com>
> > > ---
> > >  tools/perf/util/evsel.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > >
> > > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > > index d23db6755f51..d1e8862b86ce 100644
> > > --- a/tools/perf/util/evsel.c
> > > +++ b/tools/perf/util/evsel.c
> > > @@ -2446,6 +2446,13 @@ bool perf_evsel__fallback(struct evsel *evsel, int err,
> > >               char *new_name;
> > >               const char *sep = ":";
> > >
> > > +             if (evsel->core.attr.exclude_user) {
> > > +                     scnprintf(msg, msgsize,
> > > +"kernel.perf_event_paranoid=%d, event set to exclude user, so cannot also exclude kernel",
> > > +                             paranoid);
> > > +                     return false;
> >
> > I'm not able to get this error printed, it seems to be
> > overwritten by perf_evsel__open_strerror call
> >
> > please include perf example with the new output
> 
> Agreed, it is possible the change builtin-top/sched/record so that on
> error the msg is checked and dumped in verbose mode. I think it is
> also fine to just remove the scnprintf. Do you have a preference?

not sure ;-) but let's make sure we don't remove some expected output

jirka

> 
> Thanks,
> Ian
> 
> > thanks,
> > jirka
> >
> > > +             }
> > > +
> > >               /* Is there already the separator in the name. */
> > >               if (strchr(name, '/') ||
> > >                   strchr(name, ':'))
> > > --
> > > 2.26.0.110.g2183baf09c-goog
> > >
> >
>