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

[skannan]

On 2018-02-24 00:41, Peter Zijlstra 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.
>
> So why would the existing ACTIVE_PKG not work for you? Because clearly
> your example does not cross a package.

Because based on testing it on hardware, it looks like the two clusters 
in an ARM DynamIQ design are not considered part of the same "package". 
When I say clusters, I using the more common interpretation of 
"homogeneous CPUs running on the same clock"/CPUs in a cpufreq policy 
and not ARM's new redefinition of cluster. So, on a SoC with 4 little 
and 4 big cores, it'll still trigger a lot of unnecessary smp calls/IPIs 
that cause unnecessary wakeups.

Although, I like Mark's suggestion of just giving a cpumask for every 
event and using that instead. Because the meaning of "active package" is 
very ambiguous. For example if a SoC has 2 DynamIQ blocks (not sure if 
that's possible), what's considered a package? CPUs that are sitting on 
one L3 can't read the PMU counters of a different L3. In that case, 
neither "Any CPU" nor "Active Package" is correct/usable for reducing 
IPIs.

> > Signed-off-by: Saravana Kannan <skannan@xxxxxxxxxxxxxx>
> > ---
> >  include/linux/perf_event.h |  3 +++
> >  kernel/events/core.c       | 10 ++++++++--
> >  2 files changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 7546822..ee8978f 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -510,9 +510,12 @@ typedef void (*perf_overflow_handler_t)(struct 
> > perf_event *,
> >   * PERF_EV_CAP_SOFTWARE: Is a software event.
> >   * PERF_EV_CAP_READ_ACTIVE_PKG: A CPU event (or cgroup event) that 
> > can be read
> >   * from any CPU in the package where it is active.
> > + * PERF_EV_CAP_READ_ANY_CPU: A CPU event (or cgroup event) that can 
> > be read
> > + * from any CPU.
> >   */
> >  #define PERF_EV_CAP_SOFTWARE		BIT(0)
> >  #define PERF_EV_CAP_READ_ACTIVE_PKG	BIT(1)
> > +#define PERF_EV_CAP_READ_ANY_CPU	BIT(2)
> >
> >  #define SWEVENT_HLIST_BITS		8
> >  #define SWEVENT_HLIST_SIZE		(1 << SWEVENT_HLIST_BITS)
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 5d3df58..570187b 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -3484,6 +3484,10 @@ static int __perf_event_read_cpu(struct 
> > perf_event *event, int event_cpu)
> >  {
> >  	u16 local_pkg, event_pkg;
> >
> > +	if (event->group_caps & PERF_EV_CAP_READ_ANY_CPU) {
> > +		return smp_processor_id();
> > +	}
> > +
> >  	if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
> >  		int local_cpu = smp_processor_id();
>
> > @@ -3575,6 +3579,7 @@ int perf_event_read_local(struct perf_event 
> > *event, u64 *value,
> >  {
> >  	unsigned long flags;
> >  	int ret = 0;
> > +	bool is_any_cpu = !!(event->group_caps & PERF_EV_CAP_READ_ANY_CPU);
> >
> >  	/*
> >  	 * Disabling interrupts avoids all counter scheduling (context
> > @@ -3600,7 +3605,8 @@ 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 != smp_processor_id() &&
> > +	    !is_any_cpu) {
> >  		ret = -EINVAL;
> >  		goto out;
> >  	}
> > @@ -3610,7 +3616,7 @@ 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() || is_any_cpu)
> >  		event->pmu->read(event);
> >
> >  	*value = local64_read(&event->count);
>
> And why are you modifying read_local for this? That didn't support
> ACTIVE_PKG, so why should it support this?

Maybe I'll make a separate patch to first have perf_event_read_local() 
also handle ACTIVE_PACKAGE? Because in those cases, the smp call made by 
read_local is unnecessary too.

> And again, where are the users?

The DynamIQ PMU driver would be the user.

-Saravana