Re: [PATCH v2] perf/core: Add support for PMUs that can be read from more than 1 CPU

[Saravana Kannan]

On 03/05/2018 04:17 AM, Mark Rutland wrote:
> On Fri, Mar 02, 2018 at 05:14:53PM -0800, Saravana Kannan wrote:
> > Some PMUs events can be read from more than the one CPU. So allow the
> > PMU driver 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 overhead and wake ups).
> >
> > When a PMU driver marks an event as such, care must be taken by the
> > driver to make sure they can handle the event being read/updated from
> > more than 1 CPU at the same time (Eg: due to an IRQ indicating event
> > counter overflow and another thread trying to read the latest values).
> >
> > Good examples of such events would be events from caches shared across
> > CPUs.
> >
> > Signed-off-by: Saravana Kannan <skannan@xxxxxxxxxxxxxx>
> > ---
> > Changes since v1:
> > - Use cpumasks instead of capability flag as that's more flexible.
> >
> >   include/linux/perf_event.h |  1 +
> >   kernel/events/core.c       | 14 +++++++++-----
> >   2 files changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 7546822..4cec431 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -629,6 +629,7 @@ struct perf_event {
> >
> >   	int				oncpu;
> >   	int				cpu;
> > +	cpumask_t			readable_on_cpus;
>
> For most PMUs, this will be emptry, and it's potentially *very* large
> (e.g. on systems where NR_CPUS is 4096). Please use a poitner to a mask,
> as I suggested in [1], e.g.
>
> 	cpumask_t			*read_mask;
>
> That way, PMUs which already maintain an affinity mask can share that
> between all of their events.
>
> PMUs with PERF_EV_CAP_READ_ACTIVE_PKG can be updated to flip that mask
> in pmu::add() and pmu::del(). I assume there are existing sibling masks
> we can use. That means we can remove PERF_EV_CAP_READ_ACTIVE_PKG
> entriely...

I think that should be a separate patch if it's necessary.

> >   	struct list_head		owner_entry;
> >   	struct task_struct		*owner;
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 5d3df58..1a8fbfa 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -3483,10 +3483,12 @@ struct perf_read_data {
> >   static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
> >   {
> >   	u16 local_pkg, event_pkg;
> > +	int local_cpu = smp_processor_id();
> >
> > -	if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
> > -		int local_cpu = smp_processor_id();
> > +	if (cpumask_test_cpu(local_cpu, &event->readable_on_cpus))
> > +		return local_cpu;
> >
> > +	if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
> >   		event_pkg = topology_physical_package_id(event_cpu);
> >   		local_pkg = topology_physical_package_id(local_cpu);
>
> ... and this would simplify down to:
>
> 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)
> 		return local_cpu;
>
> 	return event_cpu;
> }
>
> > @@ -3575,7 +3577,8 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
> >   {
> >   	unsigned long flags;
> >   	int ret = 0;
> > -
> > +	int local_cpu = smp_processor_id();
> > +	bool readable = cpumask_test_cpu(local_cpu, &event->readable_on_cpus);
> >   	/*
> >   	 * Disabling interrupts avoids all counter scheduling (context
> >   	 * switches, timer based rotation and IPIs).
> > @@ -3600,7 +3603,8 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
> >
> >   	/* If this is a per-CPU event, it must be for this CPU */
> >   	if (!(event->attach_state & PERF_ATTACH_TASK) &&
> > -	    event->cpu != smp_processor_id()) {
> > +	    event->cpu != local_cpu &&
> > +	    !readable) {
> >   		ret = -EINVAL;
> >   		goto out;
> >   	}
> > @@ -3610,7 +3614,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
> >   	 * or local to this CPU. Furthermore it means its ACTIVE (otherwise
> >   	 * oncpu == -1).
> >   	 */
> > -	if (event->oncpu == smp_processor_id())
> > +	if (event->oncpu == smp_processor_id() || readable)
> >   		event->pmu->read(event);
>
> Please explain why you need to change perf_event_read_local().
>
> Is there a case where you have numbers to show that
> perf_event_read_local() is a bottleneck? If so, please elaborate.
>
> As-is, this doesn't seem right.

The only reason perf_event_read_local seems to exist in the first place 
is because we need an API that doesn't sleep or send IPIs to other CPUs. 
If an event is readable on the current CPU, there's no reason to sleep 
or send an IPI to another CPU. So, why disallow a read that'll work? 
Hence the change.

Thanks,
Saravana

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