linux-kernel - Re: [PATCH v7 2/7] sched: move cfs task on a CPU with higher capacity

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Date:	Thu, 9 Oct 2014 13:23:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com,
	kamalesh@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-arm-kernel@...ts.infradead.org, riel@...hat.com,
	efault@....de, nicolas.pitre@...aro.org,
	linaro-kernel@...ts.linaro.org, daniel.lezcano@...aro.org,
	dietmar.eggemann@....com, pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH v7 2/7] sched: move cfs task on a CPU with higher capacity

On Tue, Oct 07, 2014 at 02:13:32PM +0200, Vincent Guittot wrote:
> +++ b/kernel/sched/fair.c
> @@ -5896,6 +5896,18 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group)
>  }
>  
>  /*
> + * Check whether the capacity of the rq has been noticeably reduced by side
> + * activity. The imbalance_pct is used for the threshold.
> + * Return true is the capacity is reduced
> + */
> +static inline int
> +check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
> +{
> +	return ((rq->cpu_capacity * sd->imbalance_pct) <
> +				(rq->cpu_capacity_orig * 100));
> +}
> +
> +/*
>   * Group imbalance indicates (and tries to solve) the problem where balancing
>   * groups is inadequate due to tsk_cpus_allowed() constraints.
>   *
> @@ -6567,6 +6579,14 @@ static int need_active_balance(struct lb_env *env)
>  		 */
>  		if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
>  			return 1;
> +
> +		/*
> +		 * The src_cpu's capacity is reduced because of other
> +		 * sched_class or IRQs, we trig an active balance to move the
> +		 * task
> +		 */
> +		if (check_cpu_capacity(env->src_rq, sd))
> +			return 1;
>  	}

So does it make sense to first check if there's a better candidate at
all? By this time we've already iterated the current SD while trying
regular load balancing, so we could know this.


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