linux-kernel - Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher capacity

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Date:	Thu, 29 May 2014 16:04:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com, efault@....de,
	nicolas.pitre@...aro.org, linaro-kernel@...ts.linaro.org,
	daniel.lezcano@...aro.org
Subject: Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher
 capacity

On Fri, May 23, 2014 at 05:53:04PM +0200, Vincent Guittot wrote:
> @@ -7282,6 +7289,12 @@ static inline int nohz_kick_needed(struct rq *rq)
>  
>  		if (nr_busy > 1)
>  			goto need_kick_unlock;
> +
> +		if ((rq->cfs.h_nr_running >= 1)
> +		 && ((rq->cpu_power * sd->imbalance_pct) <
> +					(rq->cpu_power_orig * 100)))
> +			goto need_kick_unlock;
> +
>  	}
>  
>  	sd = rcu_dereference(per_cpu(sd_asym, cpu));

So what happens when a cpu is consistently low on power (say due to a
pinned RT task) the balancer would quickly adjust the load level, but
this would endlessly kick things into action, even though we're balanced
just fine.
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