Message-ID: <528BEB6F.8040704@kernel.dk>
Date:	Tue, 19 Nov 2013 15:51:27 -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 03:42 PM, Jens Axboe wrote:
> 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.

Patching in the old rq path works however, so...

-- 
Jens Axboe

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