linux-kernel - Re: [REGRESSION] um: rcu_sched

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Message-ID: <AANLkTimJZXL4gqq5y4eAZMqTEpN8Xy4S2B0cYANyK5qJ@mail.gmail.com>
Date:	Sat, 16 Oct 2010 17:27:36 +0200
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	penberg@...helsinki.fi, LKML <linux-kernel@...r.kernel.org>,
	user-mode-linux-devel@...ts.sourceforge.net,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Jeff Dike <jdike@...toit.com>
Subject: Re: [REGRESSION] um: rcu_sched_state detected stall on CPU 0

On Fri, Oct 15, 2010 at 9:48 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, 2010-10-15 at 10:02 +0300, Pekka Enberg wrote:
>> On Fri, Oct 15, 2010 at 2:44 AM, richard -rw- weinberger
>> <richard.weinberger@...il.com> wrote:
>> > On Thu, Oct 14, 2010 at 9:50 PM, Arjan van de Ven
>> <arjan@...ux.intel.com> wrote:
>> >>  On 10/14/2010 11:27 AM, richard -rw- weinberger wrote:
>> >>>
>> >>> Hi Arjan!
>> >>>
>> >>> This commit causes some problems on UML.
>> >>>
>> >> that is extremely weird.
>> >>>
>> >>> The kernel freezes after a few seconds until it gets some input.
>> >>> e.g: When I run top it stops refreshing the process list until i
>> press a
>> >>> button.
>> >>
>> >> a slab timer change (to not be as critical) causing global timer
>> issues....
>> >> that's very obviously not a problem with this patch.
>> >> has this been seem anywhere except UML ?
>> >
>> > A small update:
>> > It seems that CONFIG_NO_HZ is broken on UML. :-(
>> >
>> > CONFIG_NO_HZ + CONFIG_SLAB: works
>> > CONFIG_NO_HZ + CONFIG_SLAB + your patch: broken
>> > CONFIG_NO_HZ + CONFIG_SLUB: broken
>> >
>> > CONFIG_SLAB + your patch: works
>> > CONFIG_SLAB: works
>> > CONFIG_SLUB: works
>>
>> Thanks for testing! Thomas, Ingo, Peter, I'm not sure who maintains
>> CONFIG_NO_HZ so I CC'd you. The problem here is that Arjan's
>> deferrable timers patch in SLAB triggered something that looks like a
>> latent bug with UML and NOHZ.
>
> Thomas does mostly, but if its UML specific, I guess its Jeff Dike
> you'll be wanting to talk to, since he's the arch maintainer.

After reviewing the code for hours I've found the bug.
It's a int/long long issue within arch/um/os-Linux/time.c.
A patch is on the way!

-- 
Thanks,
//richard
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