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From: Rob Herring <robh@kernel.org>
To: Will Deacon <will@kernel.org>, Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@redhat.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Raphael Gault <raphael.gault@arm.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Ian Rogers <irogers@google.com>,
	honnappa.nagarahalli@arm.com,
	Itaru Kitayama <itaru.kitayama@gmail.com>
Subject: Re: [PATCH v4 2/9] arm64: perf: Enable pmu counter direct access for perf event on armv8
Date: Fri, 20 Nov 2020 14:03:45 -0600	[thread overview]
Message-ID: <20201120200345.GA1194400@robh.at.kernel.org> (raw)
In-Reply-To: <20201119191515.GA4906@willie-the-truck>

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. ;)

Rob


WARNING: multiple messages have this Message-ID
From: Rob Herring <robh@kernel.org>
To: Will Deacon <will@kernel.org>, Mark Rutland <mark.rutland@arm.com>
Cc: Ian Rogers <irogers@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Raphael Gault <raphael.gault@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	honnappa.nagarahalli@arm.com,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Itaru Kitayama <itaru.kitayama@gmail.com>,
	Jiri Olsa <jolsa@redhat.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 2/9] arm64: perf: Enable pmu counter direct access for perf event on armv8
Date: Fri, 20 Nov 2020 14:03:45 -0600	[thread overview]
Message-ID: <20201120200345.GA1194400@robh.at.kernel.org> (raw)
In-Reply-To: <20201119191515.GA4906@willie-the-truck>

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. ;)

Rob


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  reply	other threads:[~2020-11-20 20:04 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-01 14:01 [PATCH v4 0/9] libperf and arm64 userspace counter access support Rob Herring
2020-10-01 14:01 ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 1/9] arm64: pmu: Add function implementation to update event index in userpage Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 2/9] arm64: perf: Enable pmu counter direct access for perf event on armv8 Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-11-13 18:06   ` Mark Rutland
2020-11-13 18:06     ` Mark Rutland
2020-11-19 18:35     ` Rob Herring
2020-11-19 18:35       ` Rob Herring
2020-11-19 19:15     ` Will Deacon
2020-11-19 19:15       ` Will Deacon
2020-11-20 20:03       ` Rob Herring [this message]
2020-11-20 20:03         ` Rob Herring
2020-11-20 22:08         ` Rob Herring
2020-11-20 22:08           ` Rob Herring
2020-12-02 14:57         ` Rob Herring
2020-12-02 14:57           ` Rob Herring
2021-01-07  0:17           ` Rob Herring
2021-01-07  0:17             ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 3/9] tools/include: Add an initial math64.h Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 4/9] libperf: Add libperf_evsel__mmap() Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-10-14 11:05   ` Jiri Olsa
2020-10-14 11:05     ` Jiri Olsa
2020-10-16 21:39     ` Rob Herring
2020-10-16 21:39       ` Rob Herring
2020-10-19 20:15       ` Jiri Olsa
2020-10-19 20:15         ` Jiri Olsa
2020-10-20 14:38         ` Rob Herring
2020-10-20 14:38           ` Rob Herring
2020-10-20 15:35           ` Jiri Olsa
2020-10-20 15:35             ` Jiri Olsa
2020-10-20 17:11             ` Rob Herring
2020-10-20 17:11               ` Rob Herring
2020-10-21 11:24               ` Jiri Olsa
2020-10-21 11:24                 ` Jiri Olsa
2020-11-05 16:19                 ` Rob Herring
2020-11-05 16:19                   ` Rob Herring
2020-11-05 22:41                   ` Jiri Olsa
2020-11-05 22:41                     ` Jiri Olsa
2020-11-06 21:56                     ` Rob Herring
2020-11-06 21:56                       ` Rob Herring
2020-11-11 12:00                       ` Jiri Olsa
2020-11-11 12:00                         ` Jiri Olsa
2020-11-11 14:50                         ` Rob Herring
2020-11-11 14:50                           ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 5/9] libperf: tests: Add support for verbose printing Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 6/9] libperf: Add support for user space counter access Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 7/9] libperf: Add arm64 support to perf_mmap__read_self() Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 8/9] perf: arm64: Add test for userspace counter access on heterogeneous systems Rob Herring
2020-10-01 14:01   ` Rob Herring
2020-10-01 14:01 ` [PATCH v4 9/9] Documentation: arm64: Document PMU counters access from userspace Rob Herring
2020-10-01 14:01   ` Rob Herring

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