Messages in this thread | | | Date | Sun, 25 Feb 2018 14:38:03 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU |
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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.
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