linux-kernel - Re: [PATCH v3 09/13] sched/fair: Let asymmetric cpu configurations balance at wake-up

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Date:	Mon, 15 Aug 2016 16:01:34 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	sgurrappadi@...dia.com, freedom.tan@...iatek.com,
	keita.kobayashi.ym@...esas.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 09/13] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

On Mon, Aug 15, 2016 at 03:39:49PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 25, 2016 at 02:34:26PM +0100, Morten Rasmussen wrote:
> 
> 
> Because I forgot _again_, I added:
> 
> /*
>  * Disable WAKE_AFFINE in the case where task @p doesn't fit in the
>  * capacity of either the waking CPU @cpu or the previous CPU @prev_cpu.
>  *
>  * In that case WAKE_AFFINE doesn't make sense and we'll let
>  * BALANCE_WAKE sort things out.
>  */

Thanks.

> 
> > +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> > +{
> > +	long min_cap, max_cap;
> > +
> > +	min_cap = min(capacity_orig_of(prev_cpu), capacity_orig_of(cpu));
> > +	max_cap = cpu_rq(cpu)->rd->max_cpu_capacity;
> 
> There's a tiny hole here, which I'm fairly sure we don't care about. If
> @p last ran on @prev_cpu before @prev_cpu was split from @rd this
> doesn't 'work' right.

I hadn't considered that. What is 'working right' in this scenario?
Ignoring @prev_cpu as it isn't a valid option anymore?

In that case, since @prev_cpu is only used as part the min() it should
only cause min_cap to be potentially smaller than it should be, not
larger. It could lead us to let BALANCE_WAKE take over in scenarios
where select_idle_sibling() would have been sufficient, but it should
harm.

However, as you say, I'm not sure if we care that much.

Talking about @rd, I discussed with Juri and Dietmar the other week
whether the root_domain is RCU protected, and if we therefore have to
move the call to wake_cap() after the rcu_read_lock() below. I haven't
yet done thorough investigation to find the answer. Should it be
protected?

> 
> > +	/* Minimum capacity is close to max, no need to abort wake_affine */
> > +	if (max_cap - min_cap < max_cap >> 3)
> > +		return 0;
> > +
> > +	return min_cap * 1024 < task_util(p) * capacity_margin;
> > +}
> > +
> >  /*
> >   * select_task_rq_fair: Select target runqueue for the waking task in domains
> >   * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> > @@ -5389,7 +5414,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> >  
> >  	if (sd_flag & SD_BALANCE_WAKE) {
> >  		record_wakee(p);
> > -		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> > +		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> > +			      && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> >  	}
> >  
> >  	rcu_read_lock();
> > -- 
> > 1.9.1
> >