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

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2023-12-12 3:37 p.m., Ian Rogers wrote:
> On Tue, Dec 12, 2023 at 11:39 AM <kan.liang@linux.intel.com> wrote:
>>
>> 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 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@kernel.org>
>> Closes: https://lore.kernel.org/linux-perf-users/ZXNnDrGKXbEELMXV@kernel.org/
>> Reviewed-by: Ian Rogers <irogers@google.com>
>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
>> ---
>>
>> 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,
Kan

> I'm
> wondering if there is some kind of missing "hybrid-merge"
> functionality like we have for perf stat.
> 
> Thanks,
> Ian
> 
>>  tools/perf/builtin-top.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
>> index ea8c7eca5eee..cce9350177e2 100644
>> --- a/tools/perf/builtin-top.c
>> +++ b/tools/perf/builtin-top.c
>> @@ -1027,8 +1027,8 @@ static int perf_top__start_counters(struct perf_top *top)
>>
>>         evlist__for_each_entry(evlist, counter) {
>>  try_again:
>> -               if (evsel__open(counter, top->evlist->core.user_requested_cpus,
>> -                                    top->evlist->core.threads) < 0) {
>> +               if (evsel__open(counter, counter->core.cpus,
>> +                               counter->core.threads) < 0) {
>>
>>                         /*
>>                          * Specially handle overwrite fall back.
>> --
>> 2.35.1
>>