linux-kernel - Re: [PATCH v10 2/2] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest

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Date:   Fri, 25 Aug 2017 10:31:00 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     peterz@...radead.org, mingo@...nel.org, joel.opensrc@...il.com,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        kernel-team@....com
Subject: Re: [PATCH v10 2/2] sched/rt: Add support for SD_PREFER_SIBLING on
 find_lowest_rq()

On Thu, Aug 24, 2017 at 09:55:14AM -0400, Steven Rostedt wrote:
> On Thu, 24 Aug 2017 10:06:03 +0900
> Byungchul Park <byungchul.park@....com> wrote:
> 
> > Hello Steven,
> > 
> > Can I keep your reviewed-by with comments modified?
> 
> Yes, but I have one nit.
> 
> 
> > @@ -1668,9 +1691,35 @@ static int find_lowest_rq(struct task_struct *task)
> >  				return this_cpu;
> >  			}
> >  
> > -			best_cpu = cpumask_first_and(lowest_mask,
> > -						     sched_domain_span(sd));
> > +			/*
> > +			 * If a cpu being in lowest_mask & current sd &
> > +			 * ~prefer sd is valid, that becomes our choice.
> 
> I can't parse the above sentence.

I might have simplified the comment too much.

~prefer means we want to exclude the domain span having SD_PREFER_SIBLING,
since they should be considered as fallback candidates.

W/o ~prefer, a cpu in the domain might be chosen when we consider higher
domain than the domain having SD_PREFER_SIBLING, as the best cpu, not
fallback.