linux-kernel - Re: [PATCH 4/5] sched/fair: Tune down misfit nohz kicks

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Message-ID: <20190207095743.GB32494@hirez.programming.kicks-ass.net>
Date:   Thu, 7 Feb 2019 10:57:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        vincent.guittot@...aro.org, morten.rasmussen@....com,
        Dietmar.Eggemann@....com
Subject: Re: [PATCH 4/5] sched/fair: Tune down misfit nohz kicks

On Wed, Feb 06, 2019 at 05:25:31PM +0000, Valentin Schneider wrote:
> Hi,
> 
> On 06/02/2019 16:04, Peter Zijlstra wrote:
> [...]
> >> @@ -9561,6 +9573,14 @@ static void nohz_balancer_kick(struct rq *rq)
> > 
> > 	sd = rcu_dereference(rq->sd);
> > 	if (sd) {
> > 		if ((rq->cfs.h_nr_running >= 1) &&
> > 		    check_cpu_capacity(rq, sd)) {
> > 			flags = NOHZ_KICK_MASK;
> > 			goto unlock;
> >>  		}
> >>  	}
> >>  
> >> +	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
> >> +	if (sd) {
> >> +		if (check_misfit_status(rq, sd)) {
> >> +			flags = NOHZ_KICK_MASK;
> >> +			goto unlock;
> >> +		}
> >> +	}
> > 
> > So while the exact @sd to use for check_cpu_capacity() likely doesn't
> > matter; this is a 'implicit' test for actually having asym_capacity.
> > 
> 
> I did feel compelled to use the "right" @sd to have a coherent
> imbalance_pct being used - on big.LITTLE, the @sd above this hunk would be
> MC (117 imbalance_pct) whereas the sd_asym_cpucapacity one would be
> DIE (125 imbalance_pct).

Fair enough.

> > @@ -9606,6 +9614,10 @@ static void nohz_balancer_kick(struct rq
> >  
> >  	sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
> >  	if (sd) {
> > +		/*
> > +		 * When ASYM_PACKING; see if there's a more preferred CPU going
> > +		 * idle; in which case, kick the ILB to move tasks around.
> > +		 */
> 
> s/going idle/currently idle/, no?

Yah.. D'0h..

> >  		for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
> >  			if (sched_asym_prefer(i, cpu)) {
> >  				flags = NOHZ_KICK_MASK;
> >