Re: 'perf top' broken on intel hybrid systems

[Ian Rogers]

On Sat, Dec 9, 2023 at 9:26 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Sat, Dec 9, 2023, 12:14 PM Leah Neukirchen <leah@xxxxxxxx> wrote:
>>
>>
>> > The error is because the perf top always tries to open an event on the
>> > user_requested_cpus, which are all CPUs by default.
>> >
>> > Something as below should fix it. For hybrid, open a PMU event on an
>> > unsupported CPU should be error out.
>> >
>> > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
>>
>> This fixes "perf top" on my i7-1355U.  It would be great if you could
>> get this patch into upstream and stable branches.
>
>
>
> I'll try to take a look ASAP. As we don't need this for record there is some existing logic that perf top is clearly missing. We should also have a test on perf top.

So for stat and record there is a call to evlist__create_maps:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/builtin-stat.c?h=perf-tools-next#n2730
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/builtin-record.c?h=perf-tools-next#n4210

In evlist__create_maps we do perf_evlist__propagate_maps and that will:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/lib/perf/evlist.c?h=perf-tools-next#n43
```
} else if (evlist->has_user_cpus && evsel->is_pmu_core) {
/*
* User requested CPUs on a core PMU, ensure the requested CPUs
* are valid by intersecting with those of the PMU.
*/
perf_cpu_map__put(evsel->cpus);
evsel->cpus = perf_cpu_map__intersect(evlist->user_requested_cpus,
evsel->own_cpus);
```

I think the fix should be looking to do the same map set up in top as
stat and record, presumably there was a refactor and top was
overlooked. I'll try to address this and add a basic sanity test of
doing a few iterations of perf top on stdio, which would have been
sufficient to catch this.

Thanks,
Ian