linux-kernel - Re: divide error: bdi_dirty

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Message-ID: <5060AB0E.3070809@linux.vnet.ibm.com>
Date:	Tue, 25 Sep 2012 00:18:46 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...64.org>
CC:	Conny Seidel <conny.seidel@....com>, Jan Kara <jack@...e.cz>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Fengguang Wu <fengguang.wu@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <jweiner@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: divide error: bdi_dirty_limit+0x5a/0x9e

On 09/24/2012 11:49 PM, Borislav Petkov wrote:
> On Mon, Sep 24, 2012 at 08:16:50PM +0200, Conny Seidel wrote:
>> Hi,
>>
>> On Mon, 24 Sep 2012 16:36:09 +0200
>> Borislav Petkov <bp@...64.org> wrote:
>>> [ … ]
>>>
>>> Conny, would you test pls?
>>
>> Sure thing.
>> Out of ~25 runs I only triggered it once, without the patch the
>> trigger-rate is higher.
>>
>> [   55.098249] Broke affinity for irq 81
>> [   55.105108] smpboot: CPU 1 is now offline
>> [   55.311216] smpboot: Booting Node 0 Processor 1 APIC 0x11
>> [   55.333022] LVT offset 0 assigned for vector 0x400
>> [   55.545877] smpboot: CPU 2 is now offline
>> [   55.753050] smpboot: Booting Node 0 Processor 2 APIC 0x12
>> [   55.775582] LVT offset 0 assigned for vector 0x400
>> [   55.986747] smpboot: CPU 3 is now offline
>> [   56.193839] smpboot: Booting Node 0 Processor 3 APIC 0x13
>> [   56.212643] LVT offset 0 assigned for vector 0x400
>> [   56.423201] Got negative events: -25
> 
> I see it:
> 
> __percpu_counter_sum does for_each_online_cpu without doing
> get/put_online_cpus().
> 

Maybe I'm missing something, but that doesn't immediately tell me
what's the exact source of the bug.. Note that there is a hotplug
callback percpu_counter_hotcpu_callback() that takes the same
fbc->lock before updating/resetting the percpu counters of offline
CPU. So, though the synchronization is a bit weird, I don't
immediately see a problematic race condition there.

And, speaking of hotplug callbacks, on a slightly different note,
I see one defined as ratelimit_handler(), which calls
writeback_set_ratelimit() for *every single* state change in the
hotplug sequence! Is that really intentional? num_online_cpus()
changes its value only -once- for every hotplug :-)

Regards,
Srivatsa S. Bhat

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