linux-kernel - Re: [PATCH 1/2] perf stat: Fix segfault when counting armv8

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Date:   Thu, 24 Sep 2020 23:36:23 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        Li Bin <huawei.libin@...wei.com>, Wei Li <liwei391@...wei.com>,
        Ingo Molnar <mingo@...hat.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] perf stat: Fix segfault when counting armv8_pmu
 events

On Wed, Sep 23, 2020 at 10:19:00PM +0200, Jiri Olsa wrote:
> On Wed, Sep 23, 2020 at 11:15:06PM +0900, Namhyung Kim wrote:
> > I think the problem is that armv8_pmu has a cpumask,
> > and the user requested per-task events.
> > 
> > The code tried to open the event with a dummy cpu map
> > since it's not a cpu event, but the pmu has cpu map and
> > it's passed to evsel.  So there's confusion somewhere
> > whether it should use evsel->cpus or a dummy map.
> 
> you're right, I have following cpus file in pmu:
> 
>   # cat /sys/devices/armv8_pmuv3_0/cpus 
>   0-3
> 
> covering all the cpus.. and once you have cpumask/cpus file,
> you're system wide by default in current code, but we should
> not crash ;-)
> 
> I tried to cover this case in patch below and I probably broke
> some other use cases, but perhaps we could allow to open counters
> per cpus for given workload
> 
> I'll try to look at this more tomorrow

I'm thinking about a different approach, we can ignore cpu map
for the ARM cpu PMU and use the dummy, not tested ;-)

Thanks
Namhyung


diff --git a/tools/lib/perf/evlist.c b/tools/lib/perf/evlist.c
index 2208444ecb44..cfcdbd7be066 100644
--- a/tools/lib/perf/evlist.c
+++ b/tools/lib/perf/evlist.c
@@ -45,6 +45,9 @@ static void __perf_evlist__propagate_maps(struct perf_evlist *evlist,
        if (!evsel->own_cpus || evlist->has_user_cpus) {
                perf_cpu_map__put(evsel->cpus);
                evsel->cpus = perf_cpu_map__get(evlist->cpus);
+       } else if (!evsel->system_wide && perf_cpu_map__empty(evlist->cpus)) {
+               perf_cpu_map__put(evsel->cpus);
+               evsel->cpus = perf_cpu_map__get(evlist->cpus);
        } else if (evsel->cpus != evsel->own_cpus) {
                perf_cpu_map__put(evsel->cpus);
                evsel->cpus = perf_cpu_map__get(evsel->own_cpus);