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Date: Fri, 20 Nov 2020 16:08:20 -0600 From: Rob Herring <robh@...nel.org> To: Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com> Cc: 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 20, 2020 at 02:03:45PM -0600, Rob Herring wrote: > On Thu, Nov 19, 2020 at 07:15:15PM +0000, Will Deacon wrote: > > 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. > > That's good because we're also passing NULL instead of mm which would > crash. So it must be more than it's not up to date, but it's always 0. > It looks like event_mapped on x86 uses mm_cpumask(mm) which I guess was > dropped when copying this code as it didn't work... For reference, the > x86 version of this originated in commit 7911d3f7af14a6. > > I'm not clear on why we need to update pmuserenr_el0 here anyways. To > get here userspace has to mmap the event and then unmmap it. If we did > nothing, then counter accesses would not fault until the next context > switch. > > If you all have any ideas, I'm all ears. I'm not a scheduler nor perf > hacker. ;) Here's another issue that needs addressing: https://lore.kernel.org/lkml/20200821195754.20159-3-kan.liang@linux.intel.com/ Rob
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