linux-kernel - Re: [PATCH] perf evsel amd: Fix IBS error message

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Date:   Wed, 21 Jun 2023 22:38:25 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     acme@...nel.org, peterz@...radead.org, irogers@...gle.com,
        jolsa@...nel.org, adrian.hunter@...el.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        sandipan.das@....com, ananth.narayan@....com,
        santosh.shukla@....com
Subject: Re: [PATCH] perf evsel amd: Fix IBS error message

On Wed, Jun 21, 2023 at 10:28 PM Ravi Bangoria <ravi.bangoria@....com> wrote:
>
> Hi Namhyung,
>
> On 22-Jun-23 4:03 AM, Namhyung Kim wrote:
> > Hi Ravi,
> >
> > On Tue, Jun 20, 2023 at 11:24 PM Ravi Bangoria <ravi.bangoria@....com> wrote:
> >>
> >> AMD IBS can do per-process profiling[1] and is no longer restricted to
> >> per-cpu or systemwide only. Remove stale error message.
> >>
> >> Before:
> >>   $ sudo ./perf record -e "{ibs_op//,ibs_op//}" -C 0
> >>   Error:
> >>   AMD IBS may only be available in system-wide/per-cpu mode.
> >>   Try using -a, or -C and workload affinity
> >
> > It was strange that the -C option was given already.
>
> Hmm. evsel->core.system_wide is bit confusing for me. A Comment on it's
> definition says "irrespective of user requested CPUs or threads":
>
>         /*
>          * system_wide is for events that need to be on every CPU, irrespective
>          * of user requested CPUs or threads. Map propagation will set cpus to
>          * this event's own_cpus, whereby they will contribute to evlist
>          * all_cpus.
>          */
>         bool                     system_wide;
>
> So, I guess evsel->core.system_wide doesn't depend on -a / -C.

Right, you shouldn't check this flag.  IIRC It's used by Intel PT
for dummy events to get sideband events from every CPU
regardless of the target.

The proper check would be target__has_cpu() and it seems
the evsel__open_strerror() already checks that.

Thanks,
Namhyung


>
> >
> >>
> >> After:
> >>   $ sudo ./perf record -e "{ibs_op//,ibs_op//}" -C 0
> >>   Error:
> >>   The sys_perf_event_open() syscall returned with 22 (Invalid
> >>   argument) for event (ibs_op//).
> >>   /bin/dmesg | grep -i perf may provide additional information.
> >
> > It can run newer perf tools on an old kernel but the old error
> > message seems to be invalid anyway.  So I'm ok with removing it.
>
> Right.
>
> >>
> >> [1] https://git.kernel.org/torvalds/c/30093056f7b2
> >>
> >> Signed-off-by: Ravi Bangoria <ravi.bangoria@....com>
> >> ---
> >>  tools/perf/util/evsel.c | 10 +++-------
> >>  1 file changed, 3 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> >> index 356c07f03be6..65b0b70830f0 100644
> >> --- a/tools/perf/util/evsel.c
> >> +++ b/tools/perf/util/evsel.c
> >> @@ -3092,14 +3092,10 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
> >>                         return scnprintf(msg, size,
> >>         "Invalid event (%s) in per-thread mode, enable system wide with '-a'.",
> >>                                         evsel__name(evsel));
> >> -               if (is_amd(arch, cpuid)) {
> >> -                       if (is_amd_ibs(evsel)) {
> >> -                               if (evsel->core.attr.exclude_kernel)
> >> -                                       return scnprintf(msg, size,
> >> +               if (is_amd(arch, cpuid) && is_amd_ibs(evsel)) {
> >> +                       if (evsel->core.attr.exclude_kernel) {
> >> +                               return scnprintf(msg, size,
> >>         "AMD IBS can't exclude kernel events.  Try running at a higher privilege level.");
> >
> > I'm not sure if it's enough.  The IBS PMUs have CAP_NO_EXCLUDE then
> > it can't exclude user events too, right?
>
> That's correct. Let me try to fix this whole `if (is_amd())` part properly.
>
> Thanks for your feedback,
> Ravi