Re: sched: how to pick runqueue when checking task hot?

From: Peter Zijlstra
Date: Fri Jun 06 2014 - 07:35:27 EST


On Fri, Jun 06, 2014 at 01:16:23PM +0200, Mike Galbraith wrote:
> On Fri, 2014-06-06 at 12:15 +0200, Peter Zijlstra wrote:
>
> > > --- a/kernel/sched/fair.c Fri Jun 6 12:37:37 2014
> > > +++ b/kernel/sched/fair.c Fri Jun 6 14:32:34 2014
> > > @@ -5051,7 +5051,7 @@ task_hot(struct task_struct *p, u64 now)
> > > /*
> > > * Buddy candidates are cache hot:
> > > */
> > > - if (sched_feat(CACHE_HOT_BUDDY) && this_rq()->nr_running &&
> > > + if (sched_feat(CACHE_HOT_BUDDY) && task_rq(p)->nr_running &&
> > > (&p->se == cfs_rq_of(&p->se)->next ||
> > > &p->se == cfs_rq_of(&p->se)->last))
> > > return 1;
> >
> > That does appear to make more sense indeed, seeing how buddies are pairs
> > of tasks, so protecting a lone task doesn't make sense.
> >
> >
> > Mike, how did you intend this code to work?
>
> IIRC, this_rq()->nr_running was to say if we're idle, we don't care that
> it's last/next, pull it. Not sure I'm the one who did that, but could
> be, I didn't look.
>

commit f685ceacab07d3f6c236f04803e2f2f0dbcc5afb
Author: Mike Galbraith <efault@xxxxxx>
Date: Fri Oct 23 23:09:22 2009 +0200

sched: Strengthen buddies and mitigate buddy induced latencies

...

- if (sched_feat(CACHE_HOT_BUDDY) &&
+ if (sched_feat(CACHE_HOT_BUDDY) && this_rq()->nr_running &&
(&p->se == cfs_rq_of(&p->se)->next ||
&p->se == cfs_rq_of(&p->se)->last))

Yeah, was you ;-)

OK, so we want dst_rq. Thanks!

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