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

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Date:   Wed, 16 Aug 2017 11:17:36 +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 09:42:01PM -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.
> 
> BTW, "we have to get more chances" doesn't really make sense. Do you
> mean "we need to try other domains"?

Yes, we need to try other domains first if current domain is
SD_PREFER_SIBLING flaged.

> > > > +				 */
> > > > +				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?
> >
> 
> I may be the one confused here ;-)
> 
> I think I misread the patch. So, the SD_PREFER_SIBLING means to try to
> find a CPU in another sd instead? Thus, we try to find a CPU in a sd
> that does not have SD_PREFER_SIBLING set. And if there is none, we use
> the preferred sd as a fallback. Is that correct?

Yes, that's what I intended. IOW:

If (we found a proper sd, not having SD_PREFER_SIBLING?)
	use the sd;
else if (we found a proper sd, having SD_PREFER_SIBLING?)
	use the smallest sd among SD_PREFER_SIBLING sds;

Thanks,
Byungchul