On Wed, 16 Aug 2017 09:38:11 +0900
Byungchul Park <[email protected]> wrote:
> 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.
BTW, "we have to get more chances" doesn't really make sense. Do you
mean "we need to try other domains"?
> > > + */
> > > + 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?
I'm not familiar with the SD_PREFER_SIBLING flag, the only
documentation I can find about it is the comment that states:
/* Prefer to place tasks in a sibling domain */
And the very informative git commit change log:
commit b5d978e0c7e79a7ff842e895c85a86b38c71f1cd
Date: Tue Sep 1 10:34:33 2009 +0200
sched: Add SD_PREFER_SIBLING
Do the placement thing using SD flags.
;-)
-- Steve
