linux-kernel - Re: [PATCH v2 11/13] sched/fair: Avoid pulling tasks from non-overloaded higher capacity groups

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Date:	Fri, 15 Jul 2016 09:39:49 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Sai Gurrappadi <sgurrappadi@...dia.com>
Cc:	peterz@...radead.org, mingo@...hat.com, dietmar.eggemann@....com,
	yuyang.du@...el.com, vincent.guittot@...aro.org,
	mgalbraith@...e.de, linux-kernel@...r.kernel.org,
	Peter Boonstoppel <pboonstoppel@...dia.com>
Subject: Re: [PATCH v2 11/13] sched/fair: Avoid pulling tasks from
 non-overloaded higher capacity groups

On Thu, Jul 14, 2016 at 09:39:23AM -0700, Sai Gurrappadi wrote:
> On 06/30/2016 12:49 AM, Morten Rasmussen wrote:
> > On Thu, Jun 23, 2016 at 02:20:48PM -0700, Sai Gurrappadi wrote:
> >> Hi Morten,
> >>
> >> On 06/22/2016 10:03 AM, Morten Rasmussen wrote:
> >>
> >> [...]
> >>
> >>>  
> >>> +/*
> >>> + * group_smaller_cpu_capacity: Returns true if sched_group sg has smaller
> >>> + * per-cpu capacity than sched_group ref.
> >>> + */
> >>> +static inline bool
> >>> +group_smaller_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
> >>> +{
> >>> +	return sg->sgc->max_capacity * capacity_margin <
> >>> +						ref->sgc->max_capacity * 1024;
> >>> +}
> >>> +
> >>>  static inline enum
> >>>  group_type group_classify(struct sched_group *group,
> >>>  			  struct sg_lb_stats *sgs)
> >>> @@ -6892,6 +6903,19 @@ static bool update_sd_pick_busiest(struct lb_env *env,
> >>>  	if (sgs->avg_load <= busiest->avg_load)
> >>>  		return false;
> >>>  
> >>> +	if (!(env->sd->flags & SD_ASYM_CPUCAPACITY))
> >>> +		goto asym_packing;
> >>> +
> >>> +	/* Candidate sg has no more than one task per cpu and has
> >>> +	 * higher per-cpu capacity. Migrating tasks to less capable
> >>> +	 * cpus may harm throughput. Maximize throughput,
> >>> +	 * power/energy consequences are not considered.
> >>> +	 */
> >>> +	if (sgs->sum_nr_running <= sgs->group_weight &&
> >>> +	    group_smaller_cpu_capacity(sds->local, sg))
> >>> +		return false;
> >>> +
> >>> +asym_packing:
> >>
> >> What about the case where IRQ/RT work reduces the capacity of some of
> >> these bigger CPUs? sgc->max_capacity might not necessarily capture
> >> that case.
> > 
> > Right, we could possibly improve this by using min_capacity instead, but
> > we could end up allowing tasks to be pulled to lower capacity cpus just
> > because one big cpu has reduced capacity due to RT/IRQ pressure and
> > therefore has lowered the groups min_capacity.
> > 
> > Ideally we should check all the capacities, but that complicates things
> > a lot.
> > 
> > Would you prefer min_capacity instead, or attempts to consider all the
> > cpu capacities available in both groups?
> > 
> 
> min_capacity as a start works I think given that we are only trying to
> make existing LB better, not necessarily optimizing for every case.
> Might have to revisit this anyways for thermals etc.

Agreed, I will make it min_capacity instead of max_capacity in v3.

Thanks,
Morten