linux-kernel - Re: [PATCH RFC 1/3] sched: introduce distinct per-cpu load average

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Message-ID: <1349341186.4438.1.camel@twins>
Date:	Thu, 04 Oct 2012 10:59:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrea Righi <andrea@...terlinux.com>
Cc:	Paul Menage <paul@...lmenage.org>, Ingo Molnar <mingo@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 1/3] sched: introduce distinct per-cpu load average

On Thu, 2012-10-04 at 01:05 +0200, Andrea Righi wrote:
> +++ b/kernel/sched/core.c
> @@ -727,15 +727,17 @@ static void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
>  void activate_task(struct rq *rq, struct task_struct *p, int flags)
>  {
>         if (task_contributes_to_load(p))
> -               rq->nr_uninterruptible--;
> +               cpu_rq(p->on_cpu_uninterruptible)->nr_uninterruptible--;
>  
>         enqueue_task(rq, p, flags);
>  }

That's completely broken, you cannot do non-atomic cross-cpu
modifications like that. Also, adding an atomic op to the wakeup/sleep
paths isn't going to be popular at all.

>  void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
>  {
> -       if (task_contributes_to_load(p))
> -               rq->nr_uninterruptible++;
> +       if (task_contributes_to_load(p)) {
> +               task_rq(p)->nr_uninterruptible++;
> +               p->on_cpu_uninterruptible = task_cpu(p);
> +       }
>  
>         dequeue_task(rq, p, flags);
>  } 

This looks pointless, at deactivate time task_rq() had better be rq or
something is terribly broken.
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