linux-kernel - Re: [PATCH 1/2] sched: remove extraneous load manipulations

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Message-Id: <1216384754.28405.31.camel@twins>
Date:	Fri, 18 Jul 2008 14:39:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	stable@...r.kernel.org, linux-rt-users@...r.kernel.org,
	rostedt@...dmis.org, mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched: remove extraneous load manipulations

On Thu, 2008-07-03 at 15:37 -0600, Gregory Haskins wrote:
> commit 62fb185130e4d420f71a30ff59d8b16b74ef5d2b reverted some patches
> in the scheduler, but it looks like it may have left a few redundant
> calls to inc_load/dec_load remain in set_user_nice (since the
>  dequeue_task/enqueue_task take care of the load.  This could result
> in the load values being off since the load may change while dequeued.

I just checked out v2.6.25.10 but cannot see dequeue_task() do it.

deactivate_task() otoh does do it.

static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
{
	p->sched_class->dequeue_task(rq, p, sleep);
	p->se.on_rq = 0;
}

vs

static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep)
{
	if (task_contributes_to_load(p))
		rq->nr_uninterruptible++;

	dequeue_task(rq, p, sleep);
	dec_nr_running(p, rq);
}

where

static void dec_nr_running(struct task_struct *p, struct rq *rq)
{
	rq->nr_running--;
	dec_load(rq, p);
}

And since set_user_nice() actually changes the load we'd better not
forget to do this dec/inc load stuff.

So I'm thinking this patch would actually break stuff.

> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Ingo Molnar <mingo@...e.hu>
> ---
> 
>  kernel/sched.c |    6 ++----
>  1 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 31f91d9..b046754 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4679,10 +4679,8 @@ void set_user_nice(struct task_struct *p, long nice)
>  		goto out_unlock;
>  	}
>  	on_rq = p->se.on_rq;
> -	if (on_rq) {
> +	if (on_rq)
>  		dequeue_task(rq, p, 0);
> -		dec_load(rq, p);
> -	}
>  
>  	p->static_prio = NICE_TO_PRIO(nice);
>  	set_load_weight(p);
> @@ -4692,7 +4690,7 @@ void set_user_nice(struct task_struct *p, long nice)
>  
>  	if (on_rq) {
>  		enqueue_task(rq, p, 0);
> -		inc_load(rq, p);
> +
>  		/*
>  		 * If the task increased its priority or is running and
>  		 * lowered its priority, then reschedule its CPU:
> 

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