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

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Date:   Mon, 10 Sep 2018 10:13:36 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        kbuild test robot <lkp@...el.com>,
        Kernel Team <Kernel-team@...com>, Tejun Heo <tj@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>
Subject: Re: [PATCH v2 1/1] perf: Sharing PMU counters across compatible
 events

On Thu, Aug 30, 2018 at 06:35:37PM +0000, Song Liu wrote:
> 
> 
> > On Aug 30, 2018, at 8:13 AM, Jiri Olsa <jolsa@...hat.com> 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? 

I was just thinking since the pmu is disable we could
we don't need to read the event on 2 places.. it's almost
identic code

jirka