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

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Date:   Wed, 27 May 2020 12:28:05 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Jin, Yao" <yao.jin@...ux.intel.com>
Cc:     acme@...nel.org, jolsa@...nel.org, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v2 1/2] perf evlist: Ensure grouped events with same cpu
 map

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

> 
> I'm now thinking if we can solve this issue by an easy way.
> 
> Thanks
> Jin Yao
>