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

[Ian Rogers]

On Tue, Apr 14, 2020 at 8:12 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> 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@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?
>
> not sure ;-) but let's make sure we don't remove some expected output
>
> jirka

I went the easier route as I don't think the new error message adds
anything over the perf_evsel__open_strerror message. v2 is here:
https://lore.kernel.org/lkml/20200414161550.225588-1-irogers@xxxxxxxxxx/T/#u

Thanks,
Ian

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