[dm-devel] [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

[Jens Axboe]

On 1/19/18 9:37 AM, Ming Lei wrote:
> On Fri, Jan 19, 2018 at 09:27:46AM -0700, Jens Axboe wrote:
>> On 1/19/18 9:26 AM, Ming Lei wrote:
>>> On Fri, Jan 19, 2018 at 09:19:24AM -0700, Jens Axboe wrote:
>>>> On 1/19/18 9:05 AM, Ming Lei wrote:
>>>>> On Fri, Jan 19, 2018 at 08:48:55AM -0700, Jens Axboe wrote:
>>>>>> On 1/19/18 8:40 AM, Ming Lei wrote:
>>>>>>>>>> Where does the dm STS_RESOURCE error usually come from - what's exact
>>>>>>>>>> resource are we running out of?
>>>>>>>>>
>>>>>>>>> It is from blk_get_request(underlying queue), see
>>>>>>>>> multipath_clone_and_map().
>>>>>>>>
>>>>>>>> That's what I thought. So for a low queue depth underlying queue, it's
>>>>>>>> quite possible that this situation can happen. Two potential solutions
>>>>>>>> I see:
>>>>>>>>
>>>>>>>> 1) As described earlier in this thread, having a mechanism for being
>>>>>>>>    notified when the scarce resource becomes available. It would not
>>>>>>>>    be hard to tap into the existing sbitmap wait queue for that.
>>>>>>>>
>>>>>>>> 2) Have dm set BLK_MQ_F_BLOCKING and just sleep on the resource
>>>>>>>>    allocation. I haven't read the dm code to know if this is a
>>>>>>>>    possibility or not.
>>>>>>>>
>>>>>>>> I'd probably prefer #1. It's a classic case of trying to get the
>>>>>>>> request, and if it fails, add ourselves to the sbitmap tag wait
>>>>>>>> queue head, retry, and bail if that also fails. Connecting the
>>>>>>>> scarce resource and the consumer is the only way to really fix
>>>>>>>> this, without bogus arbitrary delays.
>>>>>>>
>>>>>>> Right, as I have replied to Bart, using mod_delayed_work_on() with
>>>>>>> returning BLK_STS_NO_DEV_RESOURCE(or sort of name) for the scarce
>>>>>>> resource should fix this issue.
>>>>>>
>>>>>> It'll fix the forever stall, but it won't really fix it, as we'll slow
>>>>>> down the dm device by some random amount.
>>>>>>
>>>>>> A simple test case would be to have a null_blk device with a queue depth
>>>>>> of one, and dm on top of that. Start a fio job that runs two jobs: one
>>>>>> that does IO to the underlying device, and one that does IO to the dm
>>>>>> device. If the job on the dm device runs substantially slower than the
>>>>>> one to the underlying device, then the problem isn't really fixed.
>>>>>
>>>>> I remembered that I tried this test on scsi-debug & dm-mpath over scsi-debug,
>>>>> seems not observed this issue, could you explain a bit why IO over dm-mpath
>>>>> may be slower? Because both two IO contexts call same get_request(), and
>>>>> in theory dm-mpath should be a bit quicker since it uses direct issue for
>>>>> underlying queue, without io scheduler involved.
>>>>
>>>> Because if you lose the race for getting the request, you'll have some
>>>> arbitrary delay before trying again, potentially. Compared to the direct
>>>
>>> But the restart still works, one request is completed, then the queue
>>> is return immediately because we use mod_delayed_work_on(0), so looks
>>> no such issue.
>>
>> There are no pending requests for this case, nothing to restart the
>> queue. When you fail that blk_get_request(), you are idle, nothing
>> is pending.
> 
> I think we needn't worry about that, once a device is attached to
> dm-rq, it can't be mounted any more, and usually user don't use the device
> directly and by dm-mpath at the same time.

Here's an example of that, using my current block tree (merged into
master).  The setup is dm-mpath on top of null_blk, the latter having
just a single request. Both are mq devices.

Fio direct 4k random reads on dm_mq: ~250K iops

Start dd on underlying device (or partition on same device), just doing
sequential reads.

Fio direct 4k random reads on dm_mq with dd running: 9 iops

No schedulers involved.

https://i.imgur.com/WTDnnwE.gif

-- 
Jens Axboe