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.