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

[Song Liu]

> On Dec 12, 2019, at 5:42 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> 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?

This should work. Let me fix in v9. 

Thanks,
Song