From owner-freebsd-hackers@FreeBSD.ORG  Mon May 12 06:10:52 2008
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id A41591065674
	for <freebsd-hackers@freebsd.org>; Mon, 12 May 2008 06:10:52 +0000 (UTC)
	(envelope-from julian@elischer.org)
Received: from outY.internet-mail-service.net (outy.internet-mail-service.net
	[216.240.47.248])
	by mx1.freebsd.org (Postfix) with ESMTP id 8CC228FC43
	for <freebsd-hackers@freebsd.org>; Mon, 12 May 2008 06:10:52 +0000 (UTC)
	(envelope-from julian@elischer.org)
Received: from mx0.idiom.com (HELO idiom.com) (216.240.32.160)
	by out.internet-mail-service.net (qpsmtpd/0.40) with ESMTP;
	Mon, 12 May 2008 12:44:40 -0700
Received: from julian-mac.elischer.org (localhost [127.0.0.1])
	by idiom.com (Postfix) with ESMTP id A38CD2D600D;
	Sun, 11 May 2008 23:10:51 -0700 (PDT)
Message-ID: <4827DF6B.6050101@elischer.org>
Date: Sun, 11 May 2008 23:10:51 -0700
From: Julian Elischer <julian@elischer.org>
User-Agent: Thunderbird 2.0.0.14 (Macintosh/20080421)
MIME-Version: 1.0
To: Elijah Buck <elijah.buck@gmail.com>
References: <303eddcb0805111943t7c456c79l56f427c526d0b454@mail.gmail.com>
In-Reply-To: <303eddcb0805111943t7c456c79l56f427c526d0b454@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Cc: freebsd-hackers@freebsd.org
Subject: Re: 4bsd fuzzy runq
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 12 May 2008 06:10:52 -0000

Elijah Buck wrote:
> Hi,
> 
> I'm looking at the code for 4bsd fuzzy run queues in kern_switch.c
> 
> The relevant bit:
> 
> if (fuzz > 1) {
>     int count = fuzz;
>     int cpu = PCPU_GET(cpuid);
>     struct thread *td2;
>     td2 = td = TAILQ_FIRST(rqh);
>     while (count-- && td2) {
>         if (td->td_lastcpu == cpu) {

this should be td2. The bug is mine..
and I wondered why the code commited never had the same result as that 
I tested in another tree.

>             td = td2;
>             break;
>         }
>         td2 = TAILQ_NEXT(td2, td_runq);
>     }
> ...return(td)
> The purpose of this code appears to be to look through the runq to a depth
> defined by fuzz for a thread that was last run on the current cpu.
> 
> Here are the cases I see:
> 1.) if (td_lastcpu == cpu) on the first iteration then TAILQ_FIRST(rqh) is
> the selected thread (because of the break).
> 2.) if (td_lastcpu != cpu) on the first iteration then td is never set
> again, and so (td_lastcpu != cpu) is always true. So, again,
> TAILQ_FIRST(rqh) will be selected (because count will reach 0).
> 
> Which doesn't seem right (since that's what the else clause does). What am I
> missing here?
> 
> Thanks,
> Elijah
> _______________________________________________
> freebsd-hackers@freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
> To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org"