linux-kernel - Re: [RFC v2 PATCH 4/8] sched: Enforce hard limits by throttling

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Message-Id: <1255444020.8392.362.camel@twins>
Date:	Tue, 13 Oct 2009 16:27:00 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	bharata@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>
Subject: Re: [RFC v2 PATCH 4/8] sched: Enforce hard limits by throttling

On Wed, 2009-09-30 at 18:22 +0530, Bharata B Rao wrote:

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 0f1ea4a..77ace43 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1024,7 +1024,7 @@ struct sched_domain;
>  struct sched_class {
>  	const struct sched_class *next;
>  
> -	void (*enqueue_task) (struct rq *rq, struct task_struct *p, int wakeup);
> +	int (*enqueue_task) (struct rq *rq, struct task_struct *p, int wakeup);
>  	void (*dequeue_task) (struct rq *rq, struct task_struct *p, int sleep);
>  	void (*yield_task) (struct rq *rq);
>  

I really hate this, it uglfies all the enqueue code in a horrid way
(which is most of this patch).

Why can't we simply enqueue the task on a throttled group just like rt?

> @@ -3414,6 +3443,18 @@ int can_migrate_task(struct task_struct *p, struct rq *rq, int this_cpu,
>  	}
>  
>  	/*
> +	 * Don't migrate the task if it belongs to a
> +	 * - throttled group on its current cpu
> +	 * - throttled group on this_cpu
> +	 * - group whose hierarchy is throttled on this_cpu
> +	 */
> +	if (cfs_rq_throttled(cfs_rq_of(&p->se)) ||
> +		task_group_throttled(task_group(p), this_cpu)) {
> +		schedstat_inc(p, se.nr_failed_migrations_throttled);
> +		return 0;
> +	}
> +
> +	/*
>  	 * Aggressive migration if:
>  	 * 1) task is cache cold, or
>  	 * 2) too many balance attempts have failed.

Simply don't iterate throttled groups?


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