linux-kernel - Re: [RFCv3 PATCH 44/48] sched: Tipping point from energy-aware to conventional load balancing

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Message-ID: <20150324152655.GT23123@twins.programming.kicks-ass.net>
Date:	Tue, 24 Mar 2015 16:26:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, yuyang.du@...el.com,
	preeti@...ux.vnet.ibm.com, mturquette@...aro.org, nico@...aro.org,
	rjw@...ysocki.net, juri.lelli@....com, linux-kernel@...r.kernel.org
Subject: Re: [RFCv3 PATCH 44/48] sched: Tipping point from energy-aware to
 conventional load balancing

On Wed, Feb 04, 2015 at 06:31:21PM +0000, Morten Rasmussen wrote:
> From: Dietmar Eggemann <dietmar.eggemann@....com>
> 
> Energy-aware load balancing bases on cpu usage so the upper bound of its
> operational range is a fully utilized cpu. Above this tipping point it
> makes more sense to use weighted_cpuload to preserve smp_nice.
> This patch implements the tipping point detection in update_sg_lb_stats
> as if one cpu is over-utilized the current energy-aware load balance
> operation will fall back into the conventional weighted load based one.
> 
> cc: Ingo Molnar <mingo@...hat.com>
> cc: Peter Zijlstra <peterz@...radead.org>
> 
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> ---
>  kernel/sched/fair.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6b79603..4849bad 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6723,6 +6723,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  		sgs->sum_weighted_load += weighted_cpuload(i);
>  		if (idle_cpu(i))
>  			sgs->idle_cpus++;
> +
> +		/* If cpu is over-utilized, bail out of ea */
> +		if (env->use_ea && cpu_overutilized(i, env->sd))
> +			env->use_ea = false;
>  	}

I don't immediately see why this is desired. Why would a single
overloaded CPU be reason to quit? It could be the cpus simply aren't
'balanced' right and the group as a whole is still under utilized.

In that case we want to continue the balance pass to reach this
equilibrium.
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