From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1757449Ab2AKK24 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 11 Jan 2012 05:28:56 -0500
Received: from mail-lpp01m010-f46.google.com ([209.85.215.46]:37627 "EHLO
	mail-lpp01m010-f46.google.com" rhost-flags-OK-OK-OK-OK)
	by vger.kernel.org with ESMTP id S1753678Ab2AKK2y (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 11 Jan 2012 05:28:54 -0500
Message-ID: <1326277729.2767.5.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Subject: Re: [BUG] kernel freezes with latest tree
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Ahern <dsahern@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Suresh Siddha <suresh.b.siddha@intel.com>
Date: Wed, 11 Jan 2012 11:28:49 +0100
In-Reply-To: <1326272685.2442.120.camel@twins>
References: <1326171444.6638.3.camel@edumazet-laptop>
	 <1326171798.6638.4.camel@edumazet-laptop>
	 <1326183371.6638.6.camel@edumazet-laptop>
	 <1326212033.19095.3.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
	 <CA+55aFwNVutn=Z8N9k3CHLri=EKWX2UcN8skZvMfJ9Tg1LHCpg@mail.gmail.com>
	 <1326213442.19095.9.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
	 <CA+55aFyyZUJHELdbKfk2Gzpx9np7Ov74idttCo6+wi2+MAVG=g@mail.gmail.com>
	 <1326214407.19095.11.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
	 <CA+55aFzUSMVD84acwDGC_mqBUWg-6rdy6n_BfP6up+_V3oGx7A@mail.gmail.com>
	 <1326234230.2614.15.camel@edumazet-laptop>
	 <CA+55aFwYmX7q4MZc-xOStZB2ZQjVt_Jca6qDP+0cPKekz8yL+A@mail.gmail.com>
	 <4F0D2D9B.8030501@gmail.com> <1326272685.2442.120.camel@twins>
Content-Type: text/plain; charset="UTF-8"
X-Mailer: Evolution 3.2.1- 
Content-Transfer-Encoding: 8bit
Mime-Version: 1.0
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Le mercredi 11 janvier 2012 à 10:04 +0100, Peter Zijlstra a écrit :
> On Tue, 2012-01-10 at 23:35 -0700, David Ahern wrote:
> > On 01/10/2012 04:44 PM, Linus Torvalds wrote:
> > > Anybody? Any ideas? Clearly there can be a merge problem that doesn't
> > > actually show as a real data conflict, just some semantic conflict,
> > > but I don't see what such issues would be brouht in by the scheduler
> > > merge anyway.
> > 
> > This is really easy to reproduce in a KVM hosted VM.
> > 
> > Using the gdb stub one cpu is spinning here:
> > 
> > (gdb) bt
> > #0  try_to_wake_up (p=0xf529b200, state=<optimized out>, wake_flags=1)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/core.c:1575
> > #1  0xc0470ab0 in default_wake_function (curr=<optimized out>,
> > mode=<optimized out>,
> >     wake_flags=<optimized out>, key=0xc3) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/core.c:3364
> > 
> > So basically:
> >     while (p->on_cpu) {
> > ...
> >         cpu_relax();
> >     }
> > 
> > 
> > And the other vcpu is here:
> > 
> > #0  tg_load_down (tg=0xf55b9c00, data=<optimized out>) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3351
> > #1  0xc0470049 in walk_tg_tree_from (from=0xc0ba5400, down=0xc04753c0
> > <tg_load_down>, up=0xc046a3b0 <tg_nop>,
> >     data=0x0) at /mnt/sw/kernel-2.6.git/kernel/sched/core.c:664
> > #2  0xc04793f7 in walk_tg_tree (data=<optimized out>, up=<optimized
> > out>, down=0xc04753c0 <tg_load_down>)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/sched.h:175
> > #3  update_h_load (cpu=<optimized out>) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3361
> > #4  load_balance_fair (lb_flags=<synthetic pointer>,
> > idle=CPU_NEWLY_IDLE, sd=0xf5c30800, max_load_move=278,
> >     busiest=0xf6607d00, this_cpu=1, this_rq=0xf6707d00) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3374
> > #5  move_tasks (lb_flags=<synthetic pointer>, idle=CPU_NEWLY_IDLE,
> > sd=0xf5c30800, max_load_move=278,
> >     busiest=<optimized out>, this_cpu=1, this_rq=0xf6707d00)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3444
> > #6  load_balance (this_cpu=1, this_rq=0xf6707d00, sd=0xf5c30800,
> > idle=CPU_NEWLY_IDLE, balance=0xf5217cb4)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:4496
> > #7  0xc0479be2 in idle_balance (this_cpu=1, this_rq=0xf6707d00)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:4640
> > 
> > 
> > Based on the file in question (sched/fair.c) I took a stab at guessing
> > the commit: without a195f004 I was not able to lock it up. With the
> > patch the VM spins after a few hackbench iterations.
> > 
> > I don't have time for a proper bisect tonight. I can do that in the a.m.
> > if I am not totally off base here. Peter: any chance this commit could
> > explain the spinning cpus / system freeze?
> 
> It could, I certainly ran into similar issues while developing that
> patch. I ran into all sorts of weird stuff but had hoped I'd cured all
> of it.
> 
> If Eric can confirm this is indeed what is causing his pain, I'm fine
> with reverting it and having another go at it later. How easy is it to
> reproduce using your KVM thing?
> 
> I think simply replacing the one |= LBF_NEED_BREAK with a LBF_ABORT or
> removing that condition all-together should make the hang go away.
> 
> I'll try and figure out how it ends up in the infinite retry loop after
> my brain wakes up a bit more. Maybe adding a few more NEED_BREAK bits
> and making it a counter and overflowing it into ABORT might be good.
> 
> /me off to get breakfast and morning-juice..
> 


I let my machine run 3.2 kernel for 37 minutes with no hang, so at least
we know there is no hardware glitch.

$ uptime
 11:26:45 up 37 min,  2 users,  load average: 201.80, 206.06, 170.68

I'll try now latest tree with a195f004 reverted.

Thanks