linux-kernel - Re: [RESEND PATCH 2/3 v5] sched: Rewrite per entity runnable load average tracking

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Date:	Tue, 21 Oct 2014 16:54:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org, pjt@...gle.com,
	bsegall@...gle.com, arjan.van.de.ven@...el.com,
	len.brown@...el.com, rafael.j.wysocki@...el.com,
	alan.cox@...el.com, mark.gross@...el.com, fengguang.wu@...el.com
Subject: Re: [RESEND PATCH 2/3 v5] sched: Rewrite per entity runnable load
 average tracking

On Fri, Oct 10, 2014 at 10:21:56AM +0800, Yuyang Du wrote:
>  /*
> + * Updating tg's load_avg is necessary before update_cfs_share (which is done)
> + * and effective_load (which is not done because it is too costly).
>   */
> +static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  {
> +	long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
>  
> +	if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
> +		atomic_long_add(delta, &cfs_rq->tg->load_avg);
> +		cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg;
>  	}
>  }

In the thread here: lkml.kernel.org/r/1409094682.29189.23.camel@...irtualBox
there are concerns about the error bounds of such constructs. We can
basically 'leak' nr_cpus * threshold, which is potentially a very large
number.

Do we want to introduce the force updated to combat this?

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