Re: [PATCH 2/5] libperf: Propagate maps only if necessary

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

On 27/09/22 20:28, Namhyung Kim wrote:
> Hi Adrian,
> 
> On Tue, Sep 27, 2022 at 12:06 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>
>> On 24/09/22 19:57, Namhyung Kim wrote:
>>> The current code propagate evsel's cpu map settings to evlist when it's
>>> added to an evlist.  But the evlist->all_cpus and each evsel's cpus will
>>> be updated in perf_evlist__set_maps() later.  No need to do it before
>>> evlist's cpus are set actually.
>>>
>>> Actually we discarded this intermediate all_cpus maps at the beginning
>>> of perf_evlist__set_maps().  Let's not do this.  It's only needed when
>>> an evsel is added after the evlist cpu maps are set.
>>
>> That might not be true.  Consider evlist__fix_hybrid_cpus() which fiddles
>> with evsel->core.cpus and evsel->core.own_cpus after the evsel has been
>> added to the evlist.  It can also remove an evsel from the evlist.
> 
> Thanks for your review.  I think it's fine to change evsel cpus or to remove
> an evsel from evlist before calling evlist__create_maps().  The function
> will take care of setting evlist's all_cpus from the evsels in the evlist.
> So previous changes in evsel/cpus wouldn't be any special.
> 
> After this point, adding a new evsel needs to update evlist all cpus by
> propagating cpu maps.  So I think hybrid cpus should be fine.
> Did I miss something?

I wondered how it might play out if evlist__fix_hybrid_cpus() reduced the
cpus from the target->cpu_list (using perf record -C) , since after this
patch all_cpus always starts with the target->cpu_list instead of an empty
list.  But then, in the hybrid case, it puts a dummy event that uses the
target cpu list anyway, so the result is the same.

I don't know if there are any cases where all_cpus would actually need to
exclude some of the cpus from target->cpu_list.

> 
>>
>> There might be other cases like that, but that was just one that stuck
>> out.
> 
> Thanks for sharing your concern.  Please let me know if you could
> come up with another.
> 
> Namhyung