linux-kernel - Re: Block driver freezes when using CFQ

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Date:	Tue, 31 Oct 2006 10:27:02 +0530
From:	Ravi Krishnamurthy <Ravi_Krishnamurthy@...ptec.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re:  Block driver freezes when using CFQ

Jens Axboe wrote:
> On Sat, Oct 28 2006, Ravi Krishnamurthy wrote:
>> Hi all,
>>
>>    I have written a block driver that registers a virtual device and
>> routes requests to appropriate real devices after some re-mapping of
>> the requests. I am testing the driver by creating a filesystem on the
>> virtual device and copying a large number of files on to it. The test
>> causes the device to become unresponsive after some time. After some
>> debugging, I noticed that this happens only if the I/O scheduler being
>> used is CFQ. I have not had any trouble if the scheduler is noop,
>> anticipatory or deadline. The problem occurs on all the kernels I have
>> tested - 2.6.18-rc2, 2.6.18-rc4, 2.6.19-rc3.
>>


> 
> The io scheduler is not obligated to recall your request handling
> function, _unless_ you have no pending io at the point where
> elv_next_request() returns NULL but there are things pending. 
> IOW, when you complete your requests you want to just recall your request handling
> function. Just insert something ala:
> 
>         if (elv_next_request(q))
>                 q->request_fn(q);
> 
> when you are done completing requests.
> 
> Does that fix it?

I haven't had a chance to test this fix. A workaround I had tried was to
insert these lines at the end of the request function:
        if (! elv_queue_empty(q))
             blk_plug_device(q);

This worked for me. So I assume the fix you have suggested will surely
work.

I am curious to know why the problem does not occur when I am using the
anticipatory scheduler. Also, in the suggested fix, is it guaranteed that
elv_next_request() will not return NULL as long as the elevator queue is
not empty?

Thanks,
Ravi.
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