linux-kernel - Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

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Date:	Sun, 15 Apr 2007 21:20:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Willy Tarreau <w@....eu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Nick Piggin <npiggin@...e.de>, 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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Slaby <jirislaby@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]


* Willy Tarreau <w@....eu> wrote:

> > to debug this, could you try to apply this add-on as well:
> > 
> >   http://redhat.com/~mingo/cfs-scheduler/sched-fair-print.patch
> > 
> > with this patch applied you should have a /proc/sched_debug file 
> > that prints all runnable tasks and other interesting info from the 
> > runqueue.
> 
> I don't know if you have seen my mail from yesterday evening (here). I 
> found that changing keventd prio fixed the problem. You may be 
> interested in the description. I sent it at 21:01 (+200).

ah, indeed i missed that mail - the response to the patches was quite 
overwhelming (and i naively thought people dont do Linux hacking over 
the weekends anymore ;).

so Linus was right: this was caused by scheduler starvation. I can see 
one immediate problem already: the 'nice offset' is not divided by 
nr_running as it should. The patch below should fix this but i have yet 
to test it accurately, this change might as well render nice levels 
unacceptably ineffective under high loads.

	Ingo

--------->
---
 kernel/sched_fair.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -31,7 +31,9 @@ static void __enqueue_task_fair(struct r
 	int leftmost = 1;
 	long long key;
 
-	key = rq->fair_clock - p->wait_runtime + p->nice_offset;
+	key = rq->fair_clock - p->wait_runtime;
+	if (unlikely(p->nice_offset))
+		key += p->nice_offset / rq->nr_running;
 
 	p->fair_key = key;
 
-
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