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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160630074958.GA12540@e105550-lin.cambridge.arm.com>
Date:	Thu, 30 Jun 2016 08:49:58 +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, 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?