linux-kernel - Re: [PATCH 0/1] Don't update group weights when on service tree.

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Message-ID: <4D55FAC1.9070208@cn.fujitsu.com>
Date:	Sat, 12 Feb 2011 11:13:05 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Justin TerAvest <teravest@...gle.com>
CC:	jaxboe@...ionio.com, vgoyal@...hat.com, ctalbott@...gle.com,
	mrubin@...gle.com, jmoyer@...hat.com, guijanfeng@...fujitsu.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] Don't update group weights when on service tree.

Justin TerAvest wrote:
> With some instrumentation, we can observe that the total_weight for a 
> service tree can be badly adjusted; particularly when the weight for a 
> group is adjusted without taking it off of the tree. This can be 
> reproduced on the HEAD of the linux-2.6-block tree.
> 
> We have seen this problem in workloads when total_weight becomes 0 and 
> we divide by 0 in cfq_group_slice(), crashing the kernel, but it's
> easier to illustrate by adding a BUG_ON and making it signed, like this:

Justin,

I have also catch this BUG by browsing code a few days ago. ;)

The problem is we don't update st->total_weight when we setting a new weight
for a group.

Thanks,
Gui

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