Re: [RESEND PATCH 2/2] perf/core: Allow reading package events from perf_event_read_local

[Tero Kristo]

On 12/09/2023 17:04, Peter Zijlstra wrote:
> On Tue, Sep 12, 2023 at 03:44:32PM +0300, Tero Kristo wrote:
> > Per-package perf events are typically registered with a single CPU only,
> > however they can be read across all the CPUs within the package.
> > Currently perf_event_read maps the event CPU according to the topology
> > information to avoid an unnecessary SMP call, however
> > perf_event_read_local deals with hard values and rejects a read with a
> > failure if the CPU is not the one exactly registered. Allow similar
> > mapping within the perf_event_read_local if the perf event in question
> > can support this.
> >
> > This allows users like BPF code to read the package perf events properly
> > across different CPUs within a package.
> >
> > Signed-off-by: Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx>
> > ---
> >   kernel/events/core.c | 10 +++++++---
> >   1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 4c72a41f11af..780dde646e8a 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -4528,6 +4528,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
> >   {
> >   	unsigned long flags;
> >   	int ret = 0;
> > +	int event_cpu;
> >   
> >   	/*
> >   	 * Disabling interrupts avoids all counter scheduling (context
> > @@ -4551,15 +4552,18 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
> >   		goto out;
> >   	}
> >   
> > +	event_cpu = READ_ONCE(event->oncpu);
> > +	event_cpu = __perf_event_read_cpu(event, event_cpu);
> What happens with __perf_event_read_cpu() when event_cpu == -1 ?

Good question. It looks like I need to add a check against that. Will 
update and send v2 out.

-Tero


> > +
> >   	/* 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()) {
> >   		ret = -EINVAL;
> >   		goto out;
> >   	}
> >   
> >   	/* If this is a pinned event it must be running on this CPU */
> > -	if (event->attr.pinned && event->oncpu != smp_processor_id()) {
> > +	if (event->attr.pinned && event_cpu != smp_processor_id()) {
> >   		ret = -EBUSY;
> >   		goto out;
> >   	}
> > @@ -4569,7 +4573,7 @@ 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_cpu == smp_processor_id())
> >   		event->pmu->read(event);
> >   
> >   	*value = local64_read(&event->count);
> > --
> > 2.40.1