Re: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Feb 23, 2018 at 04:19:38PM -0800, Saravana Kannan wrote:
> Some PMUs events can be read from any CPU. So allow the PMU to mark
> events as such. For these events, we don't need to reject reads or
> make smp calls to the event's CPU and cause unnecessary wake ups.
> 
> Good examples of such events would be events from caches shared across
> all CPUs.

I think that if we need to generalize PERF_EV_CAP_READ_ACTIVE_PKG, it would be
better to give events a pointer to a cpumask. That could then cover all cases
quite trivially:

static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
{
	int local_cpu = smp_processor_id();

	if (event->read_mask &&
	    cpumask_test_cpu(local_cpu, event->read_mask))
		event_cpu = local_cpu;
	
	return event_cpu;
}

... in the PERF_EV_CAP_READ_ACTIVE_PKG case, we can use the exiting(?) package
masks, and more generally we can re-use the PMU's affinit mask if it has one.

That said, I see that many pmu::read() implementations have side-effects on
hwc->prev_count, and event->count, so I worry that this won't be sfe in general
(e.g. if we race with the IRQ handler on another CPU).

Thanks,
Mark.