linux-kernel - Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle

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Message-ID: <20210108134736.GC3592@techsingularity.net>
Date:   Fri, 8 Jan 2021 13:47:36 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        valentin.schneider@....com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com,
        tim.c.chen@...ux.intel.com, benbjiang@...il.com
Subject: Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost
 accounting

On Fri, Jan 08, 2021 at 01:01:10PM +0000, Qais Yousef wrote:
> On 01/08/21 10:27, Mel Gorman wrote:
> >  	for_each_cpu_wrap(cpu, cpus, target) {
> > -		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> > +		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) {
> > +			/* Adjust cost of a successful scan */
> > +			loops <<= 2;
> > +
> >  			break;
> > +		}
> >  
> > -		if (loops >= nr) {
> > +		if (++loops >= nr) {
> >  			cpu = -1;
> >  			break;
> >  		}
> > -		loops++;
> 
> Random (out of the blue) comment.
> 
> Now this will increment loops before the comparison/break. ie: we're
> effectively doing one iteration less IIRC. Should loops be initialized to
> 0 instead of 1?
> 

Yep, although in practice it'll make little difference except after a
rapid phase change when avg_idle still appears high on a per-rq basis
yet the domain is fully busy with no idle CPUs.

-- 
Mel Gorman
SUSE Labs