linux-kernel - Re: scheduler oddity [bug?]

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Message-ID: <20090308153956.GB19658@elte.hu>
Date:	Sun, 8 Mar 2009 16:39:56 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Galbraith <efault@....de>
Cc:	Balazs Scheidler <bazsi@...abit.hu>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: scheduler oddity [bug?]


* Mike Galbraith <efault@....de> wrote:

> The problem with your particular testcase is that while one 
> half has an avg_overlap (what we use as affinity hint for 
> synchronous wakeups) which triggers the affinity hint, the 
> other half has avg_overlap of zero, what it was born with, so 
> despite significant execution overlap, the scheduler treats 
> them as if they were truly synchronous tasks.

hm, why does it stay on zero?

>  static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
>  {
> +	u64 limit = sysctl_sched_migration_cost;
> +	u64 runtime = p->se.sum_exec_runtime - p->se.prev_sum_exec_runtime;
> +
>  	if (sleep && p->se.last_wakeup) {
>  		update_avg(&p->se.avg_overlap,
>  			   p->se.sum_exec_runtime - p->se.last_wakeup);
>  		p->se.last_wakeup = 0;
> -	}
> +	} else if (p->se.avg_overlap < limit && runtime >= limit)
> +		update_avg(&p->se.avg_overlap, runtime);
>  
>  	sched_info_dequeued(p);
>  	p->sched_class->dequeue_task(rq, p, sleep);

hm, that's weird. We want to limit avg_overlap maintenance to 
true sleeps only.

And this patch only makes a difference in the !sleep case - 
which shouldnt be that common in this workload.

What am i missing? A trace would be nice of a few millisecs of 
runtime of this workload, with a few more trace_printk()'s added 
showing how avg_overlap develops.

Maybe it's some task migration artifact? There we do a 
dequeue/enqueue with sleep=0.

	Ingo
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