linux-kernel - Re: [RFC][PATCH 1/5] sched, fair: Let minimally loaded cpu balance the group

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Date:	Wed, 02 May 2012 12:31:30 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	mingo@...nel.org, pjt@...gle.com, suresh.b.siddha@...el.com,
	efault@....de, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/5] sched, fair: Let minimally loaded cpu balance
 the group

On Wed, 2012-05-02 at 15:55 +0530, Srivatsa Vaddagiri wrote:
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> [2012-05-01 20:14:31]:
> 
> > @@ -3795,12 +3796,11 @@ static inline void update_sg_lb_stats(st
> > 
> >  		/* Bias balancing toward cpus of our domain */
> >  		if (local_group) {
> > -			if (idle_cpu(i) && !first_idle_cpu) {
> > -				first_idle_cpu = 1;
> > +			load = target_load(i, load_idx);
> > +			if (load < balance_load || idle_cpu(i)) {
> > +				balance_load = load;
> 
> Let's say load_idx != 0 (ex: a busy cpu doing this load balance). In
> that case, for a idle cpu, we could return non-zero load and hence this
> would fail to select such a idle cpu? 

Yep, such is the nature of !0 load_idx.

> IOW :
> 
> 		balance_load = 0 iff idle_cpu(i) ??

I think so, even for !0 load_idx, load will only reach zero when we're
idle, just takes longer.
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