On Fri, 2014-06-06 at 13:34 +0200, Peter Zijlstra wrote:
> 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 <efa...@gmx.de>
> 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 ;-)
Last hunk prevents buddies from stymieing BALANCE_NEWIDLE via
CACHE_HOT_BUDDY.
Last hunk, first hunk, whatever, that's what it was for :)
-Mike
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