Re: [PATCH v10] perf: Sharing PMU counters across compatible events

[Peter Zijlstra]

On Fri, Feb 28, 2020 at 10:36:04AM +0100, Peter Zijlstra wrote:
> +
> +		/*
> +		 * Flip an active event to a new master; this is tricky because
> +		 * for an active event event_pmu_read() can be called at any
> +		 * time from NMI context.
> +		 *
> +		 * This means we need to have ->dup_master and
> +		 * ->dup_count consistent at all times. Of course we cannot do
> +		 * two writes at once :/
> +		 *
> +		 * Instead, flip ->dup_master to EVENT_TOMBSTONE, this will
> +		 * make event_pmu_read_dup() NOP. Then we can set
> +		 * ->dup_count and finally set ->dup_master to the new_master
> +		 * to let event_pmu_read_dup() rip.
> +		 */
> +		WRITE_ONCE(tmp->dup_master, EVENT_TOMBSTONE);
> +		barrier();
> +
> +		count = local64_read(&new_master->count);
> +		local64_set(&tmp->dup_count, count);
> +
> +		if (tmp == new_master)
> +			local64_set(&tmp->master_count, count);
> +
> +		barrier();
> +		WRITE_ONCE(tmp->dup_master, new_master);
>  		dup_count++;

> @@ -4453,12 +4484,14 @@ static void __perf_event_read(void *info
>  
>  static inline u64 perf_event_count(struct perf_event *event)
>  {
> -	if (event->dup_master == event) {
> -		return local64_read(&event->master_count) +
> -			atomic64_read(&event->master_child_count);
> -	}
> +	u64 count;
>  
> -	return local64_read(&event->count) + atomic64_read(&event->child_count);
> +	if (likely(event->dup_master != event))
> +		count = local64_read(&event->count);
> +	else
> +		count = local64_read(&event->master_count);
> +
> +	return count + atomic64_read(&event->child_count);
>  }
>  
>  /*

One thing that I've failed to mention so far (but has sorta been implied
if you thought carefully) is that ->dup_master and ->master_count also
need to be consistent at all times. Even !ACTIVE events can have
perf_event_count() called on them.

Worse; I just realize that perf_event_count() is called remotely, so we
need SMP ordering between reading ->dup_master and ->master_count
*groan*....