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

[skannan]

On 2018-02-27 03:43, Mark Rutland wrote:
> On Mon, Feb 26, 2018 at 06:11:45PM -0800, skannan@xxxxxxxxxxxxxx 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