Re: [PATCH] perf top: Use evsel's cpus to replace user_requested_cpus

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Dec 12, 2023 at 10:00:16AM -0800, Ian Rogers wrote:
> On Tue, Dec 12, 2023 at 9:23 AM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Tue, Dec 12, 2023 at 7:56 AM 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 escreveu:
> > > >> 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 hybrid
> > > >> 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 ¯\_(ツ)_/¯.

On ARM it'll be essentially the same as on x86: if you open an event with
type==PERF_EVENT_TYPE_HARDWARE (without the extended HW type pointing to a
specific PMU), and with cpu==-1, it'll go to an arbitrary CPU PMU, whichever
happens to be found by perf_init_event() when iterating over the 'pmus' list.

If you open an event with type==PERF_EVENT_TYPE_HARDWARE and cpu!=-1, the event
will opened on the appropriate CPU PMU, by virtue of being rejected by others
when perf_init_event() iterates over the 'pmus' list.

Mark.