Re: [PATCH V2] perf top: Use evsel's cpus to replace user_requested_cpus

[Namhyung Kim]

On Tue, Dec 12, 2023 at 2:12 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Tue, Dec 12, 2023 at 1:25 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> >
> >
> >
> > On 2023-12-12 3:37 p.m., Ian Rogers wrote:
> > > On Tue, Dec 12, 2023 at 11:39 AM <kan.liang@xxxxxxxxxxxxxxx> wrote:
> > >>
> > >> 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 perf top expects that the "cycles" is collected on all CPUs in the
> > >> system. But for hybrid there is no single "cycles" event which can cover
> > >> all CPUs. Perf has to split it into two cycles events, e.g.,
> > >> cpu_core/cycles/ and cpu_atom/cycles/. Each event has its own CPU mask.
> > >> If a event is opened on the unsupported CPU. The open fails. That's the
> > >> reason of the above error out.
> > >>
> > >> Perf should only open the cycles event on the corresponding CPU. The
> > >> commit ef91871c960e ("perf evlist: Propagate user CPU maps intersecting
> > >> core PMU maps") intersect the requested CPU map with the CPU map of the
> > >> PMU. Use the evsel's cpus to replace user_requested_cpus.
> > >>
> > >> The evlist's threads are also propagated to the evsel's threads in
> > >> __perf_evlist__propagate_maps(). For a system-wide event, perf appends
> > >> a dummy event and assign it to the evsel's threads. For a per-thread
> > >> event, the evlist's thread_map is assigned to the evsel's threads. The
> > >> same as the other tools, e.g., perf record, using the evsel's threads
> > >> when opening an event.
> > >>
> > >> Reported-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > >> Closes: https://lore.kernel.org/linux-perf-users/ZXNnDrGKXbEELMXV@xxxxxxxxxx/
> > >> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> > >> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > >> ---
> > >>
> > >> Changes since V1:
> > >> - Update the description
> > >> - Add Reviewed-by from Ian
> > >
> > > Thanks Kan, quick question. Does "perf top" on hybrid ask the user to
> > > select between the cycles event on cpu_atom and cpu_core?
> >
> > Yes, but the event doesn't include the PMU information.
> > We probably need a follow up patch to append the PMU name.
> >
> > Available samples
> > 385 cycles:P
> >
> > 903 cycles:P
>
> Thanks and agreed, it isn't possible to tell which is which PMU/CPU
> type at the moment. I tried the patch with perf top --stdio, there
> wasn't a choice of event  and I can't tell what counter is being
> displayed. When I quit I also see:
> ```
> exiting.
> corrupted double-linked list
> Aborted (core dumped)
> ```
> but I wasn't able to repro this on a debuggable binary/system.
>
> If my memory serves there was a patch where perf top was showing >1
> event. It would be nice here to do some kind of hybrid merging rather
> than having to view each PMU's top separately.

Using event groups, but I noticed you removed the --group option.
Maybe perf top can just use `{ ... }` notation for explicit grouping,
but it might be implicit like in the hybrid case.

Thanks,
Namhyung