linux-kernel - Re: [Fwd: Block driver freezes when using CFQ]

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Date:	Mon, 30 Oct 2006 09:22:33 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Ravi Krishnamurthy <Ravi_Krishnamurthy@...ptec.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [Fwd: Block driver freezes when using CFQ]

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.
> 
> Below are some details about the driver and what I have observed during
> testing:
> 
> The request function registered by my driver is a simple loop -
> 
>   while ((req = elv_next_request(q))) {
>         blkdev_dequeue_request(req);
> 
>         /*
>          Add request to an internal queue for further processing
>          Wake up thread to start processing the queue
>          Update some variables for book-keeping
>          */
>   }
> 
> Completed requests are handled in a different thread -
>   while (work to be done) {
>       /*
>         Dequeue completed requests from internal queue
>         Call end_that_request_first() and end_that_request_last()
>         Update some variables for book-keeping
>       */
>   }

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?

-- 
Jens Axboe

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