linux-kernel - Re: [PATCH 1/2 v2] sched: fix find_idlest

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Message-ID: <20161128170124.GA20785@codeblueprint.co.uk>
Date:   Mon, 28 Nov 2016 17:01:24 +0000
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, Morten.Rasmussen@....com,
        dietmar.eggemann@....com, kernellwp@...il.com, yuyang.du@...el.com,
        umgwanakikbuti@...il.com
Subject: Re: [PATCH 1/2 v2] sched: fix find_idlest_group for fork

On Fri, 25 Nov, at 04:34:32PM, Vincent Guittot wrote:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index aa47589..820a787 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5463,13 +5463,19 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>  	 * utilized systems if we require spare_capacity > task_util(p),
>  	 * so we allow for some task stuffing by using
>  	 * spare_capacity > task_util(p)/2.
> +	 * spare capacity can't be used for fork because the utilization has
> +	 * not been set yet as it need to get a rq to init the utilization
>  	 */
> +	if (sd_flag & SD_BALANCE_FORK)
> +		goto no_spare;
> +
>  	if (this_spare > task_util(p) / 2 &&
>  	    imbalance*this_spare > 100*most_spare)
>  		return NULL;
>  	else if (most_spare > task_util(p) / 2)
>  		return most_spare_sg;
>  
> +no_spare:
>  	if (!idlest || 100*this_load < imbalance*min_load)
>  		return NULL;
>  	return idlest;

It's only a minor comment, but would you be opposed to calling this
label 'skip_spare' to indicate that spare capacity may exist, but
we're not going to make use of it?