Re: [PATCH v2 1/2] perf evlist: Ensure grouped events with same cpu map

[Jin, Yao]

Hi Jiri,

On 5/27/2020 6:28 PM, Jiri Olsa wrote:
> On Wed, May 27, 2020 at 02:31:03PM +0800, Jin, Yao wrote:
>
> SNIP
>
> > > Thanks
> > > Jin Yao
> >
> > Issue is found!
> >
> > It looks we can't set "pos->leader = pos" in either for_each_group_member()
> > or in for_each_group_evsel() because it may exit the iteration immediately.
> >
> > 	evlist__for_each_entry(evlist, evsel) {
> > 		if (evsel->leader == evsel)
> > 			continue;
> >
> > 		if (cpu_maps_matched(evsel->leader, evsel))
> > 			continue;
> >
> > 		pr_warning("WARNING: event cpu maps are not fully matched, "
> > 			   "disable group\n");
> >
> > 		for_each_group_member(pos, evsel->leader) {
> > 			pos->leader = pos;
> > 			pos->core.nr_members = 0;
> > 		}
> >
> > Let me use the example of '{cycles,unc_cbo_cache_lookup.any_i}' again.
> >
> > In evlist:
> > cycles,
> > unc_cbo_cache_lookup.any_i,
> > unc_cbo_cache_lookup.any_i,
> > unc_cbo_cache_lookup.any_i,
> > unc_cbo_cache_lookup.any_i,
> >
> > When we reach the for_each_group_member at first time, evsel is the first
> > unc_cbo_cache_lookup.any_i and evsel->leader is cycles. pos is same as the
> > evsel (the first unc_cbo_cache_lookup.any_i).
> >
> > Once we execute "pos->leader = pos;", it's actually "evsel->leader = evsel".
> > So now evsel->leader is changed to the first unc_cbo_cache_lookup.any_i.
> >
> > In next iteration, pos is the second unc_cbo_cache_lookup.any_i. pos->leader
> > is cycles but unfortunately evsel->leader has been changed to the first
> > unc_cbo_cache_lookup.any_i. So iteration stops immediately.
>
> hum, AFAICS the iteration will not break but continue to next evsel and
> pass the 'continue' for another group member.. what do I miss?
>
> jirka

Let me use this example again.

cycles,
unc_cbo_cache_lookup.any_i,
unc_cbo_cache_lookup.any_i,
unc_cbo_cache_lookup.any_i,
unc_cbo_cache_lookup.any_i,

Yes, once for_each_group_member breaks (due to the issue in 'pos->leader = pos'), 
evlist__for_each_entry will continue to the second unc_cbo_cache_lookup.any_i. But now evsel->leader 
!= evsel (evsel->leader is "cycles"), so it will go to cpu_maps_matched.

But actually we don't need to go to cpu_maps_matched again.

for_each_group_member(pos, evsel->leader) {
	pos->leader = pos;
	pos->core.nr_members = 0;
}

If we solve the issue in above code, for_each_group_member doesn't break, the leaders of all members 
in this group will be set to themselves.

if (evsel->leader == evsel)
	continue;

So the iteration will continue to the next evsel.

Thanks
Jin Yao