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From:   Ian Rogers <irogers@google.com>
Date:   Tue, 12 Dec 2023 10:00:16 -0800
Message-ID: <CAP-5=fVd-0aSovYVsOmTo2dfKb5_PHz1KV7ePipi35_JbfJ6qQ@mail.gmail.com>
Subject: Re: [PATCH] perf top: Use evsel's cpus to replace user_requested_cpus
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        mark.rutland@arm.com, maz@kernel.org, marcan@marcan.st,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org
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On Tue, Dec 12, 2023 at 9:23=E2=80=AFAM Namhyung Kim <namhyung@kernel.org> =
wrote:
>
> On Tue, Dec 12, 2023 at 7:56=E2=80=AFAM Liang, Kan <kan.liang@linux.intel=
.com> wrote:
> >
> >
> >
> > On 2023-12-11 4:13 p.m., Arnaldo Carvalho de Melo wrote:
> > > Em Fri, Dec 08, 2023 at 01:08:55PM -0800, kan.liang@linux.intel.com e=
screveu:
> > >> From: Kan Liang <kan.liang@linux.intel.com>
> > >>
> > >> perf top errors out on a hybrid machine
> > >>  $perf top
> > >>
> > >>  Error:
> > >>  The cycles:P event is not supported.
> > >>
> > >> The user_requested_cpus may contain CPUs that are invalid for a hybr=
id
> > >> PMU. It causes perf_event_open to fail.
> > >
> > > ?
> > >
> > > All perf top expects is that the "cycles", the most basic one, be
> > > collected, on all CPUs in the system.
> > >
> >
> > Yes, but for hybrid there is no single "cycles" event which can cover
> > all CPUs.
>
> Does that mean the kernel would reject the legacy "cycles" event
> on hybrid CPUs?

I believe not. When the extended type isn't set on legacy cycles we
often have the CPU and from that can determine the PMU. The issue is
with the -1 any CPU perf_event_open option. As I was told, the PMU the
event is opened on in this case is the first one registered in the
kernel, on Intel hybrid this could be cpu_core or cpu_atom.. but IIRC
it'll probably be cpu_core. On ARM =C2=AF\_(=E3=83=84)_/=C2=AF. In any case=
 you only
have an event that will be enabled on a fraction of the CPU cores the
thread you are monitoring could be scheduled on. This is why 2 or more
events are needed (depending on how many core types you have).

Thanks,
Ian

> Thanks,
> Namhyung