Re: [PATCH V2 21/23] libperf evsel: Add comments for booleans

[Adrian Hunter <adrian.hunter@intel.com>]

On 12/05/22 08:34, Ian Rogers wrote:
> On Wed, May 11, 2022 at 12:03 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>
>> On 6/05/22 23:51, Ian Rogers wrote:
>>> On Fri, May 6, 2022 at 5:26 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>>
>>>> Add comments for 'system_wide' and 'requires_cpu' booleans
>>>>
>>>> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
>>>> ---
>>>>  tools/lib/perf/include/internal/evsel.h | 9 +++++++++
>>>>  1 file changed, 9 insertions(+)
>>>>
>>>> diff --git a/tools/lib/perf/include/internal/evsel.h b/tools/lib/perf/include/internal/evsel.h
>>>> index 77fbb8b97e5c..cc8f1ba790bd 100644
>>>> --- a/tools/lib/perf/include/internal/evsel.h
>>>> +++ b/tools/lib/perf/include/internal/evsel.h
>>>> @@ -49,7 +49,16 @@ struct perf_evsel {
>>>>
>>>>         /* parse modifier helper */
>>>>         int                      nr_members;
>>>> +       /*
>>>> +        * system_wide is for events that need to be on every CPU, irrespective
>>>> +        * of user requested CPUs or threads. Map propagation will not override
>>>> +        * this events own_cpus, which will contribute to evlist all_cpus.
>>>
>>> So this muddies my understanding of cpus and own_cpus (sigh, again
>>> undocumented). I believe cpus to be the set of CPUs (or any CPU, aka
>>> dummy) that perf_event_open happens on. all_cpus in evlist is the
>>> union of all the evsels cpus (ie not related to own_cpus as described
>>> here). Own_cpus I believe to be what's computed at parse-event time
>>> from sysfs. Is this a typo in the comment or quite likely my error?
>>
>> A 'system_wide' event is not created by the parser, so own_cpus is set
>> by the code adding the event.
> 
> I think I may be misreading the comment. Perhaps it should read:
> 
> system_wide is for events that need to be on every CPU, irrespective
> of user requested CPUs or threads. Map propagation will not override
> this event's own_cpus and own_cpus don't contribute to the evlist
> all_cpus.

For a system_wide evsel, evsel->cpus are set to own_cpus and the
evsel->cpus contribute to all_cpus.

> 
> It would be nice if there were a comment on own_cpus to explain its
> relationship to cpus and more broadly when it is used. Fwiw, the parse
> time copy is made here:
> https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/parse-events.c?h=perf/core#n367
> 
> I worry that if own_cpus should be used in place of cpus then
> iterators like evlist__for_each_cpu may need to handle this, and so
> they'd be broken currently.

No, evsel->own_cpus may be used to determine evsel->cpus, but that
is where their influence stops.

> 
> Thanks,
> Ian
> 
>>>
>>> Thanks,
>>> Ian
>>>
>>>> +        */
>>>>         bool                     system_wide;
>>>> +       /*
>>>> +        * Some events, for example uncore events, require a CPU.
>>>> +        * i.e. it cannot be the 'any CPU' value of -1.
>>>> +        */
>>>>         bool                     requires_cpu;
>>>>         int                      idx;
>>>>  };
>>>> --
>>>> 2.25.1
>>>>
>>