linux-kernel - Re: KVM bug, git bisected

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Message-ID: <4BB0D500.1090409@redhat.com>
Date:	Mon, 29 Mar 2010 12:27:44 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Kent Overstreet <kent.overstreet@...il.com>
CC:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: KVM bug, git bisected

On 03/29/2010 12:11 PM, Rik van Riel wrote:
> On 03/27/2010 08:43 AM, Kent Overstreet wrote:
>> commit 5beb49305251e5669852ed541e8e2f2f7696c53e
>> Author: Rik van Riel <riel@...hat.com>
>> Date: Fri Mar 5 13:42:07 2010 -0800
>>
>> mm: change anon_vma linking to fix multi-process server scalability issue
>>
>> I get this when starting kvm. The warning hasn't caused me problems, but
>> I've also been getting a scheduling while atomic panic when I start kvm
>> that I can only reproduce when I don't want to. It's definitely config
>> dependent, I'd guess preempt might have something to do with it.
>
>  From your trace, it looks like mm_take_all_locks is taking close
> to 256 locks, which is where the preempt_count could overflow into
> the softirq count.
>
> Since kvm-qemu is exec'd, I am guessing you have a very large
> number of VMAs in your qemu process. Is that correct?

I just took a look at a qemu-kvm process on my own system.

It has a staggering 385 VMAs!

That definately has the potential of overflowing PREEMPT_BITS
when undergoing mm_take_all_locks...

> Peter, would it be safe to increase PREEMPT_BITS to eg. 10?

Looks like we'll have to.   At least on 64 bits...
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