Re: [PATCH] perf/x86: Consider pinned events for group validation

[Liang, Kan]

On 8/20/2019 10:10 AM, Peter Zijlstra wrote:
> On Fri, Aug 16, 2019 at 10:49:10AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> >
> > perf stat -M metrics relies on weak groups to reject unschedulable
> > groups and run them as non-groups.
> > This uses the group validation code in the kernel. Unfortunately
> > that code doesn't take pinned events, such as the NMI watchdog, into
> > account. So some groups can pass validation, but then later still
> > never schedule.
>
> But if you first create the group and then a pinned event it 'works',
> which is inconsistent and makes all this timing dependent.

I don't think so. The pinned event will be validated by 
validate_event(), which doesn't simulate the schedule.
So the validation still pass, but the group still never schedule.

> > @@ -2011,9 +2011,11 @@ static int validate_event(struct perf_event *event)
> >    */
> >   static int validate_group(struct perf_event *event)
> >   {
> > +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> >   	struct perf_event *leader = event->group_leader;
> >   	struct cpu_hw_events *fake_cpuc;
> > -	int ret = -EINVAL, n;
> > +	struct perf_event *pinned_event;
> > +	int ret = -EINVAL, n, i;
> >   
> >   	fake_cpuc = allocate_fake_cpuc();
> >   	if (IS_ERR(fake_cpuc))
> > @@ -2033,6 +2035,24 @@ static int validate_group(struct perf_event *event)
> >   	if (n < 0)
> >   		goto out;
> >   
> > +	/*
> > +	 * The new group must can be scheduled
> > +	 * together with current pinned events.
> > +	 * Otherwise, it will never get a chance
> > +	 * to be scheduled later.
>
> That's wrapped short; also I don't think it is sufficient; what if you
> happen to have a pinned event on CPU1 (and not others) and happen to run
> validation for a new CPU1 event on CPUn ?

The patch doesn't support this case. It is mentioned in the description.
The patch doesn't intend to catch all possible cases that cannot be 
scheduled. I think it's impossible to catch all cases.
We only want to improve the validate_group() a little bit to catch some 
common cases, e.g. NMI watchdog interacting with group.


> Also; per that same; it is broken, you're accessing the cpu-local cpuc
> without serialization.

Do you mean accessing all cpuc serially?
We only check the cpuc on current CPU here. It doesn't intend to access 
other cpuc.


Thanks,
Kan

> > +	 */
> > +	for (i = 0; i < cpuc->n_events; i++) {
> > +		pinned_event = cpuc->event_list[i];
> > +		if (WARN_ON_ONCE(!pinned_event))
> > +			continue;
> > +		if (!pinned_event->attr.pinned)
> > +			continue;
> > +		fake_cpuc->n_events = n;
> > +		n = collect_events(fake_cpuc, pinned_event, false);
> > +		if (n < 0)
> > +			goto out;
> > +	}
> > +
> >   	fake_cpuc->n_events = 0;
> >   	ret = x86_pmu.schedule_events(fake_cpuc, n, NULL);
> >   
> > --
> > 2.7.4