linux-kernel - Re: [patch] CFS (Completely Fair Scheduler), v2

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Date:	Tue, 17 Apr 2007 10:41:53 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Gene Heskett <gene.heskett@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [patch] CFS (Completely Fair Scheduler), v2

On Tue, Apr 17, 2007 at 10:26:31AM +0200, Ingo Molnar wrote:
> 
> * Nick Piggin <npiggin@...e.de> wrote:
> 
> > Actually I think this is something that makes sense to add, even if 
> > just for debugging, but maybe also for production, depending on how 
> > much it impacts things. Child runs first is an heuristic optimisation 
> > that exploits a VM detail (however fundamental). But for things that 
> > don't exec right after forking (and maybe some things that do), it can 
> > be nicer to reduce context switches, improve cache patterns, and allow 
> > children to be load balanced away before touching memory, if 
> > child_runs_first is turned off.
> 
> yeah, the primary intent was debug. Nick, am i confused to conclude that 
> mainline in fact runs the _parent_ first, despite all the elaborate 
> runqueue juggling we do there? This piece of code in wake_up_new_task() 
> caught my eyes:
> 
>                                 p->prio = current->prio;
>                                 p->normal_prio = current->normal_prio;
>                                 list_add_tail(&p->run_list, &current->run_list);
>                                 p->array = current->array;
>                                 p->array->nr_active++;
>                                 inc_nr_running(p, rq);
> 
> shouldnt the list_add_tail() be list_add(), so that task pickup sees the 
> child first? Maybe we still do child-runs-first in practice, due to the 
> timeslice and sleep average fixups that happen if the parent preempts, 
> but the above piece of code seems a quite elaborate way of doing 
> activate_task(). To have the child _before_ the parent we'd need the 
> add-on patch below. But ... i could be wrong, this is just a quick 
> thought.

I think that it works because the list we're adding to is not the
normal runqueue list head, but the parent's list_head on that runqueue.
Which adds the child directly ahead of the parent... I think?

> 
> 	Ingo
> 
> ---
>  kernel/sched.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux/kernel/sched.c
> ===================================================================
> --- linux.orig/kernel/sched.c
> +++ linux/kernel/sched.c
> @@ -1685,7 +1685,7 @@ void fastcall wake_up_new_task(struct ta
>  			else {
>  				p->prio = current->prio;
>  				p->normal_prio = current->normal_prio;
> -				list_add_tail(&p->run_list, &current->run_list);
> +				list_add(&p->run_list, &current->run_list);
>  				p->array = current->array;
>  				p->array->nr_active++;
>  				inc_nr_running(p, rq);
-
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