linux-kernel - Re: [PATCH v4 2/9] arm64: perf: Enable pmu counter direct access for perf event on armv8

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Date:   Thu, 19 Nov 2020 19:15:15 +0000
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Rob Herring <robh@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Raphael Gault <raphael.gault@....com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Ian Rogers <irogers@...gle.com>, honnappa.nagarahalli@....com,
        Itaru Kitayama <itaru.kitayama@...il.com>
Subject: Re: [PATCH v4 2/9] arm64: perf: Enable pmu counter direct access for
 perf event on armv8

On Fri, Nov 13, 2020 at 06:06:33PM +0000, Mark Rutland wrote:
> On Thu, Oct 01, 2020 at 09:01:09AM -0500, Rob Herring wrote:
> > +static void armv8pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
> > +{
> > +	if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> > +		return;
> > +
> > +	if (atomic_dec_and_test(&mm->context.pmu_direct_access))
> > +		on_each_cpu_mask(mm_cpumask(mm), refresh_pmuserenr, NULL, 1);
> > +}
> 
> I didn't think we kept our mm_cpumask() up-to-date in all cases on
> arm64, so I'm not sure we can use it like this.
> 
> Will, can you confirm either way?

We don't update mm_cpumask() as the cost of the atomic showed up in some
benchmarks I did years ago and we've never had any need for the thing anyway
because out TLB invalidation is one or all.

Will