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

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Date:	Fri, 6 Jun 2014 13:34:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <mgalbraith@...e.de>
Cc:	zhdxzx@...a.com, linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, dhillf <dhillf@...il.com>,
	"hillf.zj" <hillf.zj@...baba-inc.com>
Subject: Re: sched: how to pick runqueue when checking task hot?

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@....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 ;-)

OK, so we want dst_rq. Thanks!


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