linux-kernel - Re: problem with "sched: revert back to per-rq vruntime"?

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Date:	Wed, 31 Dec 2008 14:17:10 -0600
From:	Jayson King <dev@...sonking.com>
To:	Mike Galbraith <efault@....de>
CC:	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl, mingo@...e.hu
Subject: Re: problem with "sched: revert back to per-rq vruntime"?

Mike Galbraith wrote:
> On Tue, 2008-12-30 at 13:37 -0600, Jayson King wrote:
>   
>> Here's some times using bash, for example. Let me know if there are some 
>> other tests I can do.
>>
>>     
>> [jayson@...as tmp]$ time bash -i -c 'exit'
>> exit
>>
>> real    0m0.753s
>> user    0m0.011s
>> sys     0m0.011s
>>     
>
> Yeah, it's b0rked.  The problem is that we call sched_slice() before the
> new task's load weight has been added to the runqueue.  
>
> Normally this doesn't cause horrible problems, but when there is a +nice
> hog on the runqueue, the lighter the hog, and/or heavier the victim...
>
> [  160.489402] bash:5858 vslice: 3840000000 weight: 1024 rq_weight: 15 inv_weight: 268435456
>
> ...the closer the victim's vruntime gets to the middle of next week.
>
> Not-signed-off diag hack.
>
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 5ad4440..3254213 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -449,13 +449,34 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  }
>  
>  /*
> + * A freshly forked task's load weight has not yet been added to the runqueue,
> + * so we can't call calc_delta_weight() and get sane results.  Therefore, we roll
> + * our own so place_entity() can place this task in the right spot.  The runqueue
> + * weight and nr_running will be updated when the task is enqueued.
> + */
> +static u64 sched_initial_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> +	u64 slice = __sched_period(cfs_rq->nr_running + 1);
> +
> +	for_each_sched_entity(se) {
> +		struct load_weight lw = cfs_rq->load;
> +
> +		lw.weight += se->load.weight;
> +		lw.inv_weight = 0;
> +		slice = calc_delta_mine(slice, se->load.weight, &lw);
> +	}
> +
> +	return slice;
> +}
> +
> +/*
>   * We calculate the vruntime slice of a to be inserted task
>   *
>   * vs = s/w
>   */
>  static u64 sched_vslice(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -	return calc_delta_fair(sched_slice(cfs_rq, se), se);
> +	return calc_delta_fair(sched_initial_slice(cfs_rq, se), se);
>  }
>  
>  /*
>   

I tested it. Now the times are back to normal.

# 2.6.28 under 100% nice CPU and Mike Galbraith patch
[jayson@...as tmp]$ time bash -i -c 'exit'
exit

real    0m0.042s
user    0m0.006s
sys     0m0.003s



Jayson

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