linux-kernel - Re: [PATCH 4/8] sched: clean up move_task() and move_one

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Date:	Wed, 20 Mar 2013 16:33:55 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] sched: clean up move_task() and move_one_task()

On Tue, Mar 19, 2013 at 03:30:15PM +0100, Peter Zijlstra wrote:
> On Thu, 2013-02-14 at 14:48 +0900, Joonsoo Kim wrote:
> > Some validation for task moving is performed in move_tasks() and
> > move_one_task(). We can move these code to can_migrate_task()
> > which is already exist for this purpose.
> 
> > @@ -4011,18 +4027,7 @@ static int move_tasks(struct lb_env *env)
> >  			break;
> >  		}
> >  
> > -		if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
> > -			goto next;
> > -
> > -		load = task_h_load(p);
> > -
> > -		if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed)
> > -			goto next;
> > -
> > -		if ((load / 2) > env->imbalance)
> > -			goto next;
> > -
> > -		if (!can_migrate_task(p, env))
> > +		if (!can_migrate_task(p, env, false, &load))
> >  			goto next;
> >  
> >  		move_task(p, env);
> 
> Right, so I'm not so taken with this one. The whole load stuff really
> is a balance heuristic that's part of move_tasks(), move_one_task()
> really doesn't care about that.
> 
> So why did you include it? Purely so you didn't have to re-order the
> tests? I don't see any reason not to flip a tests around.

I think that I'm not fully understand what you are concerning, because of
my poor English. If possible, please elaborate on a problem in more detail.

First of all, I do my best to answer your question.

Patch 4/8, 5/8 are for mitigating useless redoing overhead caused
by LBF_ALL_PINNED. For this purpose, we should check 'cpu affinity'
before evaluating a load. Just moving up can_migrate_task() above
load evaluation code may raise side effect, because can_migrate_task() have
other checking which is 'cache hottness'. I don't want a side effect. So
embedding load evaluation to can_migrate_task() and re-order checking and
makes load evaluation disabled for move_one_task().

If your recommandation is to move up can_mirate_task() above
load evaluation code, yes, I can, and will do that. :)

Please let me know what I am misunderstand.

Thanks.

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