linux-kernel - Re: [PATCH] perf tools: unbreak perf record for arm/arm64

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Date:   Mon, 9 Oct 2017 20:02:28 +0100
From:   Will Deacon <will.deacon@....com>
To:     Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        jolsa@...nel.org, stable@...r.kernel.org,
        Adrian Hunter <adrian.hunter@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        David Ahern <dsahern@...il.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] perf tools: unbreak perf record for arm/arm64

On Mon, Oct 09, 2017 at 04:00:00PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Oct 09, 2017 at 11:22:00AM +0100, Will Deacon escreveu:
> > On Fri, Oct 06, 2017 at 07:38:22PM +0100, Mark Rutland wrote:
> > > Currently, perf record is broken on arm/arm64 systems when the PMU is
> > > specified explicitly as part of the event, e.g.
> > > 
> > > $ ./perf record -e armv8_cortex_a53/cpu_cycles/u true
> > > 
> > > In such cases, perf record fails to open events unless
> > > perf_event_paranoid is set to -1, even if the PMU in question supports
> > > mode exclusion. Further, even when perf_event_paranoid is toggled, no
> > > samples are recorded.
> > > 
> > > This is an unintended side effect of commit:
> > > 
> > >   e3ba76deef23064f ("perf tools: Force uncore events to system wide monitoring)
> > > 
> > > ... which assumes that if a PMU has an associated cpu_map, it is an
> > > uncore PMU, and forces events for such PMUs to be system-wide.
> > > 
> > > This is not true for arm/arm64 systems, which can have heterogeneous
> > > CPUs. To account for this, multiple CPU PMUs are exposed, each with a
> > > "cpus" field under sysfs, which the perf tool parses into a cpu_map. ARM
> > > PMUs do not have a "cpumask" file, and only have a "cpus" file. For the
> > > gory details as to why, see commit:
> > > 
> > >  7e3fcffe95544010 ("perf pmu: Support alternative sysfs cpumask")
> > > 
> > > Given all of this, we can instead identify uncore PMUs by explicitly
> > > checking for a "cpumask" file, and restore arm/arm64 PMU support back to
> > > a working state. This patch does so, adding a new perf_pmu::is_uncore
> > > field, and splitting the existing cpumask parsing so that it can be
> > > reused.
> > > 
> > > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > > Fixes: e3ba76deef23064f ("perf tools: Force uncore events to system wide monitoring)
> > 
> > It sucks that we haven't noticed this being broken for so long, but I can
> > confirm that this fixes the issue:
> > 
> > Acked-by: Will Deacon <will.deacon@....com>
> > Tested-by Will Deacon <will.deacon@....com>
> > 
> > Any chance we can get this into 4.14? You'll probably need to do some stable
> > backports too, since this is a bit spread out.
> 
> Sure, I've added this to my perf/urgent branch, that, together with a
> few other fixes is undergoing testing now.

Thanks, Arnaldo.

Will