Re: [RFC PATCH 09/23] sched/fair: Use task-class performance score to pick the busiest group

From: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Ricardo Neri <ricardo.neri@intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Len Brown <len.brown@intel.com>, Mel Gorman <mgorman@suse.de>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Valentin Schneider <vschneid@redhat.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	"Tim C . Chen" <tim.c.chen@intel.com>
Subject: Re: [RFC PATCH 09/23] sched/fair: Use task-class performance score to pick the busiest group
Date: Thu, 6 Oct 2022 12:07:58 -0700	[thread overview]
Message-ID: <20221006190758.GA1624@ranerica-svr.sc.intel.com> (raw)
In-Reply-To: <Yz6T4OslXxKdAlM/@hirez.programming.kicks-ass.net>

On Thu, Oct 06, 2022 at 10:37:52AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 05, 2022 at 04:38:41PM -0700, Ricardo Neri wrote:
> > On Tue, Sep 27, 2022 at 01:01:32PM +0200, Peter Zijlstra wrote:
> > > On Fri, Sep 09, 2022 at 04:11:51PM -0700, Ricardo Neri wrote:
> 
> > > > @@ -9049,6 +9111,12 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> > > >  		/* Prefer to move from lowest priority CPU's work */
> > > >  		if (sched_asym_prefer(sg->asym_prefer_cpu, sds->busiest->asym_prefer_cpu))
> > > >  			return false;
> > > > +
> > > > +		/* @sg and @sds::busiest have the same priority. */
> > > > +		if (sched_asym_class_pick(sds->busiest, sg, &sds->busiest_stat, sgs))
> > > > +			return false;
> > > > +
> > > > +		/* @sg has lower priority than @sds::busiest. */
> > > >  		break;
> > > >  
> > > >  	case group_misfit_task:
> > > 
> > > So why does only this one instance of asym_prefer() require tie
> > > breaking?
> > 
> > This is the only place in which two sched groups with running tasks and of
> > equal priority are compared.
> > 
> > In all other places sched_asym_prefer() is used to compare the destination
> > CPU with others. Since asym_packing is done only when the destination CPU is
> > idle, there is no need to break this tie.

> 
> That would make for a fine comment, no? Because as presented one is left
> wondering, why if asym_prefer() needs tie breaking, only this one site
> needs it.

Sure.  I will add this comment.

> 
> > > And while looking through this, I must ask about the comment that goes
> > > with sched_set_itmt_core_prio() vs the sg->asym_prefer_cpu assignment in
> > > init_sched_groups_capacity(), what-up ?!
> > 
> > Are you referring to this comment?
> > 
> > 	"No need to rebuild sched domain after updating
> > 	 the CPU priorities. The sched domains have no
> > 	 dependency on CPU priorities"
> > 
> > If yes, then it looks wrong to me. Sched domains are rebuilt after updating
> > priorities.

I can included in the series a patch removing this comment.

Thanks and BR,
Ricardo