Messages in this thread | | | Date | Tue, 27 Feb 2018 15:15:46 -0800 | From | skannan@codeauro ... | Subject | Re: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU |
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On 2018-02-27 03:43, Mark Rutland wrote: > On Mon, Feb 26, 2018 at 06:11:45PM -0800, skannan@codeaurora.org wrote: >> On 2018-02-25 06:38, Mark Rutland wrote: >> > 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; >> > } >> >> This is a good improvement on my attempt. If I send a patch for this, >> is >> that something you'd be willing to incorporate into your patch set and >> make >> sure the DSU pmu driver handles it correctly? > > As I commented, I don't think that willl work without more invasive > changes as the DSU PMU's pmu::read() function has side effects on > hwc->prev_count and event_count, and could race with an IRQ handler on > another CPU. > > Is the IPI really a problem in practice? >
There are a bunch of cases, but the simplest one is if you try to collect DSU stats (for analysis) while measuring power, it completely messes up the power measurements.
Thanks, Saravana
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