Re: [PATCH v8] perf: Sharing PMU counters across compatible events

[Peter Zijlstra]

On Fri, Dec 06, 2019 at 04:24:47PM -0800, Song Liu wrote:
> +/* Remove dup_master for the event */
> +static void perf_event_remove_dup(struct perf_event *event,
> +				  struct perf_event_context *ctx)
> +
> +{
> +	struct perf_event *tmp, *new_master;
> +	int count;
> +
> +	/* no sharing */
> +	if (!event->dup_master)
> +		return;
> +
> +	WARN_ON_ONCE(event->state != PERF_EVENT_STATE_INACTIVE &&
> +		     event->state != PERF_EVENT_STATE_OFF);
> +
> +	/* this event is not the master */
> +	if (event->dup_master != event) {
> +		event->dup_master = NULL;
> +		return;
> +	}
> +
> +	/* this event is the master */
> +	perf_event_exit_dup_master(event);
> +
> +	count = 0;
> +	new_master = NULL;
> +	list_for_each_entry(tmp, &ctx->event_list, event_entry) {
> +		WARN_ON_ONCE(tmp->state > PERF_EVENT_STATE_INACTIVE);
> +		if (tmp->dup_master == event) {
> +			count++;
> +			if (!new_master)
> +				new_master = tmp;
> +		}
> +	}
> +
> +	if (!count)
> +		return;
> +
> +	if (count == 1) {
> +		/* no more sharing */
> +		new_master->dup_master = NULL;
> +		return;
> +	}
> +
> +	perf_event_init_dup_master(new_master);
> +
> +	/* switch to new_master */
> +	list_for_each_entry(tmp, &ctx->event_list, event_entry)
> +		if (tmp->dup_master == event)
> +			tmp->dup_master = new_master;
> +}

I'm thinking you can do that in a single iteration:

	list_for_each_entry(tmp, &ctx->event_list, event_entry) {
		if (tmp->dup_master != event)
			continue;

		if (!new_master)
			new_master = tmp;

		tmp->dup_master = new_master;
		count++;
	}

	if (count == 1)
		new_master->dup_master = NULL;
	else
		perf_event_init_dup_master(new_master);

Hmm?