linux-kernel - Re: [PATCH] blk-thrttole: Do not use kblockd workqueue for throtl work

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Date:	Mon, 28 Feb 2011 12:32:08 -0500
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...nel.org" <stable@...nel.org>,
	"dk@...telegence.net" <dk@...telegence.net>,
	"tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH] blk-thrttole: Do not use kblockd workqueue for throtl
 work

On 2011-02-28 10:51, Vivek Goyal wrote:
> o Dominik Klein reported a system hang issue while doing some blkio throttling
>   testing.
> 
>   https://lkml.org/lkml/2011/2/24/173
> 
> o Some tracing revealed that CFQ was not dispatching any more jobs as queue
>   unplug was not happening. And queue unplug was not happening because unplug
>   work was not being called as there was one throttling work on same cpu
>   which as not finished yet. And throttling work had not finished as it
>   was tyring to dispatch a bio to CFQ but all the request descriptors were
>   consume to it was put to sleep.
> 
> o So basically it is a cyclic dependecny between CFQ unplug work and throtl
>   dispatch work. Tejun suggested that use separate workqueue for such cases..
> 
> o This patch uses a separate workqueue for throttle related work and does not
>   rely on kblockd workqueue anymore.

Not good, that's definitely a bug. I'll queue this up for 2.6.38.

-- 
Jens Axboe

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