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

[Namhyung Kim]

On Tue, Dec 12, 2023 at 10:31 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> 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@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Dec 12, 2023 at 7:56 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx escreveu:
> > > > >> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > > > >>
> > > > >> 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.

Ok, that means "cycles" with cpu == -1 would not work well.

I'm curious if it's possible to do some basic work at the event_init()
like to preserve (common) resource and to do some other work at
sched to config PMU on the current CPU.  So that users can simply
use "cycles" or "instructions" for their processes.

Thanks,
Namhyung