linux-kernel - Re: [PATCH v6 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later

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Message-ID: <20170816003810.GO20323@X58A-UD3R>
Date:   Wed, 16 Aug 2017 09:38:11 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        kernel-team@....com
Subject: Re: [PATCH v6 1/2] sched/deadline: Add support for SD_PREFER_SIBLING
 on find_later_rq()

On Tue, Aug 15, 2017 at 11:19:40AM -0400, Steven Rostedt wrote:
> > @@ -1385,6 +1407,17 @@ static int find_later_rq(struct task_struct *task)
> >  			 * already under consideration through later_mask.
> >  			 */
> >  			if (best_cpu < nr_cpu_ids) {
> > +				/*
> > +				 * If current domain is SD_PREFER_SIBLING
> > +				 * flaged, we have to get more chances to
> > +				 * check other siblings.
> > +				 */
> > +				if (sd->flags & SD_PREFER_SIBLING) {
> > +					prefer = sd;
> 
> Is this how the SD_PREFER_SIBLING works? According to this, the
> preferred sd is the next sd in for_each_domain(). Not to mention, the
> prefer variable stays set if the next domain has no available CPUs. Is
> that what we want?

Maybe I don't understand what you want to say. The variable, prefer, is
used to pick up the smallest sched domain among SD_PREFER_SIBLING
domains, if more than one SD_PREFER_SIBLING domain exist in the visit.

The prefer variable alway points to the previous SD_PREFER_SIBLING domain.
And that must stay set to be used as a fallback choise if the next domain
has no available CPUs.

Could you explain what I mis-understand?

Thanks,
Byungchul

> -- Steve
> 
> 
> > +					if (fallback_cpu == -1)
> > +						fallback_cpu = best_cpu;
> > +					continue;
> > +				}
> >  				rcu_read_unlock();
> >  				return best_cpu;
> >  			}
> > @@ -1393,6 +1426,13 @@ static int find_later_rq(struct task_struct *task)
> >  	rcu_read_unlock();
> >  
> >  	/*
> > +	 * If fallback_cpu is valid, all our guesses failed *except* for
> > +	 * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
> > +	 */
> > +	if (fallback_cpu != -1)
> > +		return fallback_cpu;
> > +
> > +	/*
> >  	 * At this point, all our guesses failed, we just return
> >  	 * 'something', and let the caller sort the things out.
> >  	 */