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

[Ian Rogers]

On Tue, Apr 14, 2020 at 6:02 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> On Mon, Apr 13, 2020 at 04:55:15PM -0700, Ian Rogers wrote:
> > From: Stephane Eranian <eranian@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  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?

Thanks,
Ian

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