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

[skannan@codeaurora.org]

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