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

    Thanks,
    Ian

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

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