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

[Peter Zijlstra]

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.

> @@ -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 ?

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

> +	 */
> +	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
>