[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:

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+	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:

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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.