linux-kernel - Re: [RFC PATCH 1/7] arm64/perf: Basic uncore counter support for Cavium ThunderX

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Date:	Mon, 15 Feb 2016 14:46:52 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Jan Glauber <jan.glauber@...iumnetworks.com>
Cc:	Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 1/7] arm64/perf: Basic uncore counter support for
 Cavium ThunderX

On Mon, Feb 15, 2016 at 02:27:27PM +0000, Mark Rutland wrote:
> > > > +	uncore = event_to_thunder_uncore(event);
> > > > +	if (!uncore)
> > > > +		return -ENODEV;
> > > > +	if (!uncore->event_valid(event->attr.config))
> > > > +		return -EINVAL;
> > > > +
> > > > +	hwc->config = event->attr.config;
> > > > +	hwc->idx = -1;
> > > > +
> > > > +	/* and we don't care about CPU */
> > > 
> > > Actually, you do. You want the perf core to serialize accesses via the
> > > same CPU, so all events _must_ be targetted at the same CPU. Otherwise
> > > there are a tonne of problems you don't even want to think about.
> > 
> > I found that perf added the events on every CPU in the system. Because
> > the uncore events are not CPU related I wanted to avoid this. Setting
> > cpumask to -1 did not work. Therefore I added a single CPU in the
> > cpumask, see thunder_uncore_attr_show_cpumask().
> 
> I understand that, which is why I wrote:
> 
> > > You _must_ ensure this kernel-side, regardless of what the perf tool
> > > happens to do.
> > > 
> > > See the arm-cci and arm-ccn drivers for an example.
> 
> Take a look at drivers/bus/arm-cci.c; specifically, what we do in
> cci_pmu_event_init and cci_pmu_cpu_notifier.
> 
> This is the same thing that's done for x86 system PMUs. Take a look at
> uncore_pmu_event_init in arch/x86/kernel/cpu/perf_event_intel_uncore.c.

I note that we still have an open TODO rather than a call to
perf_pmu_migrate_context.

The better example is arm_ccn_pmu_cpu_notifier in drivers/bus/arm-ccn.c.

Mark.