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

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Date:	Tue, 17 Apr 2007 10:26:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
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


* 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.

	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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