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

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Message-ID: <20200925210149.GA3336227@krava>
Date:   Fri, 25 Sep 2020 23:01:49 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Namhyung Kim <namhyung@...nel.org>
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, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/2] perf stat: Fix segfault when counting armv8_pmu
 events

On Thu, Sep 24, 2020 at 11:36:23PM +0900, Namhyung Kim wrote:
> 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);

but I like this fix, because mine messes up scaling ;-)

>         } else if (evsel->cpus != evsel->own_cpus) {
>                 perf_cpu_map__put(evsel->cpus);
>                 evsel->cpus = perf_cpu_map__get(evsel->own_cpus);

it looks like that cpus (/sys/devices/armv8_pmuv3_0/cpus) file
can have some cpus switched off.. from brief scan of:

  drivers/perf/arm_pmu_platform.c (search for supported_cpus)

but it looks like it's just check for interrupt, so counting
might still work..?

thanks,
jirka