linux-kernel - Re: [PATCH 1/2] sched: fix and clean up calculate

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Message-ID: <20140729145308.GV12054@laptop.lan>
Date:	Tue, 29 Jul 2014 16:53:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, vincent.guittot@...aro.org,
	mikey@...ling.org, mingo@...nel.org, jhladky@...hat.com,
	ktkhai@...allels.com, tim.c.chen@...ux.intel.com,
	nicolas.pitre@...aro.org
Subject: Re: [PATCH 1/2] sched: fix and clean up calculate_imbalance

On Tue, Jul 29, 2014 at 04:49:52PM +0200, Peter Zijlstra wrote:
> > @@ -6247,32 +6247,15 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> >  		return fix_small_imbalance(env, sds);
> >  	}
> >  
> > -	if (!busiest->group_imb) {
> > -		/*
> > -		 * Don't want to pull so many tasks that a group would go idle.
> > -		 * Except of course for the group_imb case, since then we might
> > -		 * have to drop below capacity to reach cpu-load equilibrium.
> > -		 */
> > -		load_above_capacity =
> > -			(busiest->sum_nr_running - busiest->group_capacity_factor);
> > -
> > -		load_above_capacity *= (SCHED_LOAD_SCALE * SCHED_CAPACITY_SCALE);
> > -		load_above_capacity /= busiest->group_capacity;
> > -	}
> 
> I think we want to retain that, esp. for the overloaded case. So that
> wants to be:
> 
> 	if (busiest->sum_nr_running > busiest->group_capacity_factor)
> 
> Clearly it doesn't make sense for the !overload case, and we explicitly
> want to avoid it in the imb case.

Ah, wait, I think I see why you want that gone. I was only expecting a
correction fix wrt changing pick_busiest(), not also behaviour changes.

Lemme reconsider.
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