Re: [PATCH v2 1/1] perf: Sharing PMU counters across compatible events

[Song Liu]

> On Aug 30, 2018, at 8:13 AM, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> 
> On Wed, Aug 15, 2018 at 10:03:13AM -0700, Song Liu wrote:
> 
> SNIP
> 
>> 
>> +	perf_event_remove_dup(event, ctx);
>> 	/*
>> 	 * We can have double detach due to exit/hot-unplug + close.
>> 	 */
>> @@ -1982,6 +2123,92 @@ event_filter_match(struct perf_event *event)
>> 	       perf_cgroup_match(event) && pmu_filter_match(event);
>> }
>> 
>> +/* PMU sharing aware version of event->pmu->add() */
>> +static int event_pmu_add(struct perf_event *event,
>> +			 struct perf_event_context *ctx)
>> +{
>> +	struct perf_event_dup *dup;
>> +	int ret;
>> +
>> +	/* no sharing, just do event->pmu->add() */
>> +	if (event->dup_id == -1)
>> +		return event->pmu->add(event, PERF_EF_START);
>> +
>> +	dup = &ctx->dup_events[event->dup_id];
>> +
>> +	if (dup->active_event_count) {
>> +		/* already enabled */
>> +		dup->active_event_count++;
>> +		dup->master->pmu->read(dup->master);
>> +		event->dup_base_count = dup_read_count(dup);
>> +		event->dup_base_child_count = dup_read_child_count(dup);
>> +		return 0;
>> +	}
>> +
>> +	/* try add master */
>> +	ret = event->pmu->add(dup->master, PERF_EF_START);
>> +
>> +	if (!ret) {
>> +		dup->active_event_count = 1;
>> +		event->pmu->read(dup->master);
>> +		event->dup_base_count = dup_read_count(dup);
>> +		event->dup_base_child_count = dup_read_child_count(dup);
> 
> should you read the base before calling pmu->add ?
> should be same for any dup event not just master
> 
> jirka

I am not sure I am following. The pmu is disabled when we call
event_pmu_add(). Why do we need to read before calling pmu->add()? 
And this is the first added dup event for this master, so we don't
need to worry about others. 

Does this make sense? 

Thanks for the review!
Song