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

[Jens Axboe]

On 1/19/18 4:52 PM, Ming Lei wrote:
> On Fri, Jan 19, 2018 at 10:38:41AM -0700, Jens Axboe wrote:
>> 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
> 
> If null_blk's timer mode is used with a bit delay introduced, I guess
> the effect from direct access to underlying queue shouldn't be so
> serious. But it still won't be good as direct access.

Doesn't matter if it's used at the default of 10usec completion
latency, or inline complete. Same result, I ran both.

> Another way may be to introduce a variants blk_get_request(), such as
> blk_get_request_with_notify(), then pass the current dm-rq's hctx to
> it, and use the tag's waitqueue to handle that. But the change can be
> a bit big.

Yes, that's exactly the solution I suggested both yesterday and today.

-- 
Jens Axboe