Date:	Tue, 19 Nov 2013 15:42:47 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Dave Chinner <david@...morbit.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [Regression x2, 3.13-git] virtio block mq hang, iostat busted
 on virtio devices

On 11/19/2013 02:43 PM, Jens Axboe wrote:
> On 11/19/2013 02:34 PM, Dave Chinner wrote:
>> On Tue, Nov 19, 2013 at 02:20:42PM -0700, Jens Axboe wrote:
>>> On Tue, Nov 19 2013, Jens Axboe wrote:
>>>> On Tue, Nov 19 2013, Dave Chinner wrote:
>>>>> Hi Jens,
>>>>>
>>>>> I was just running xfstests on a 3.13 kernel that has had the block
>>>>> layer changed merged into it. generic/269 on XFS is hanging on a 2
>>>>> CPU VM using virtio,cache=none for the block devices under test,
>>>>> with many (130+) threads stuck below submit_bio() like this:
>>>>>
>>>>>  Call Trace:
>>>>>   [<ffffffff81adb1c9>] schedule+0x29/0x70
>>>>>   [<ffffffff817833ee>] percpu_ida_alloc+0x16e/0x330
>>>>>   [<ffffffff81759bef>] blk_mq_wait_for_tags+0x1f/0x40
>>>>>   [<ffffffff81758bee>] blk_mq_alloc_request_pinned+0x4e/0xf0
>>>>>   [<ffffffff8175931b>] blk_mq_make_request+0x3bb/0x4a0
>>>>>   [<ffffffff8174d2b2>] generic_make_request+0xc2/0x110
>>>>>   [<ffffffff8174e40c>] submit_bio+0x6c/0x120
>>>>>
>>>>> reads and writes are hung, both data (direct and buffered) and
>>>>> metadata.
>>>>>
>>>>> Some IOs are sitting in io_schedule, waiting for IO completion (both
>>>>> buffered and direct IO, both reads and writes) so it looks like IO
>>>>> completion has stalled in some manner, too.
>>>>
>>>> Can I get a recipe to reproduce this? I haven't had any luck so far.
>>>
>>> OK, I reproduced it. Looks weird, basically all 64 commands are in
>>> flight, but haven't completed. So the next one that comes in just sits
>>> there forever. I can't find any sysfs debug entries for virtio, would be
>>> nice to inspect its queue as well...
>>
>> Does it have anything to do with the fact that the request queue
>> depth is 128 entries and the tag pool only has 66 tags in it? i.e:
>>
>> /sys/block/vdb/queue/nr_requests
>> 128
>>
>> /sys/block/vdb/mq/0/tags
>> nr_tags=66, reserved_tags=2, batch_move=16, max_cache=32
>> nr_free=0, nr_reserved=1
>>   cpu00: nr_free=0
>>   cpu01: nr_free=0
>>
>> Seems to imply that if we queue up more than 66 IOs without
>> dispatching them, we'll run out of tags. And without another IO
>> coming through, the "none" scheduler that virtio uses will never
>> get a trigger to push out the currently queued IO?
> 
> No, the nr_requests isn't actually relevant in the blk-mq context, the
> driver sets its own depth. For the above, it's 64 normal commands, and 2
> reserved. The reserved would be for a flush, for instance. If someone
> attempts to queue more than the allocated number of requests, it'll stop
> the blk-mq queue and kick things into gear on the virtio side. Then when
> requests complete, we start the queue again.
> 
> If you look at virtio_queue_rq(), that handles a single request. This
> request is already tagged at this point. If we can't add it to the ring,
> we simply stop the queue and kick off whatever pending we might have. We
> return BLK_MQ_RQ_QUEUE_BUSY to blk-mq, which tells that to back off on
> sending us more. When we get the virtblk_done() callback from virtio, we
> end the requests on the blk-mq side and restart the queue.

I added some debug code to see if we had anything pending on the blk-mq
side, and it's all empty. It really just looks like we are missing
completions on the virtio side. Very odd.

-- 
Jens Axboe

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