Re: Block driver freezes when using CFQ

[Ravi Krishnamurthy <Ravi_Krishnamurthy@adaptec.com>]

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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