linux-kernel - Re: [PATCH 5/5] sched/fair: Merge select_idle

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Date:   Mon, 18 Jan 2021 14:41:19 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Qais Yousef <qais.yousef@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] sched/fair: Merge select_idle_core/cpu()

On Mon, Jan 18, 2021 at 08:55:03PM +0800, Li, Aubrey wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 12e08da90024..6c0f841e9e75 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6006,6 +6006,14 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
> >  	return new_cpu;
> >  }
> >  
> > +static inline int __select_idle_cpu(struct task_struct *p, int core, struct cpumask *cpus)
> 
> Sorry if I missed anything, why p and cpus are needed here?
> 

They are not needed. The original code was matching the calling pattern
for select_idle_core() which needs p and cpus to check if sibling CPUs
are allowed.

> > @@ -6135,7 +6147,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >  
> >  	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> >  
> > -	if (sched_feat(SIS_PROP)) {
> > +	if (sched_feat(SIS_PROP) && !smt) {
>
> Is it possible the system does have a idle core, but I still don't want to scan the entire llc domain?
> 

This version is matching historical behaviour. To limit the scan for cores,
select_idle_core() would need to obey SIS_PROP and that patch was dropped
as it introduced regressions. It would only be considered once SIS_PROP
had better metrics for limiting the depth of the search.

> >  		u64 avg_cost, avg_idle, span_avg;
> >  
> >  		/*
> > @@ -6159,16 +6171,29 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >  	for_each_cpu_wrap(cpu, cpus, target) {
> >  		if (!--nr)
> >  			return -1;
> 
> It looks like nr only makes sense when smt = false now, can it be moved into else branch below?
> 

It can. I expect the saving to be marginal and it will need to move back
when/if select_idle_core() obeys SIS_PROP.

> > -		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> > -			break;
> > +		if (smt) {
> > +			i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > +			if ((unsigned int)i < nr_cpumask_bits)
> > +				return i;
> 
> What if the last idle core is selected here, should we set_idle_cores false before return?
> 

We'd have to check what bits were still set in the cpus mask and
determine if they represent an idle core. I severely doubt it would be
worth the cost given that the availability of idle cores can change at
any instant.

-- 
Mel Gorman
SUSE Labs