[dm-devel] [PATCH] dm rq: Avoid that request processing stalls sporadically

[Jens Axboe]

On 1/18/18 5:35 PM, Ming Lei wrote:
> On Thu, Jan 18, 2018 at 05:24:29PM -0700, Jens Axboe wrote:
>> On 1/18/18 5:18 PM, Ming Lei wrote:
>>> On Fri, Jan 19, 2018 at 12:14:24AM +0000, Bart Van Assche wrote:
>>>> On Fri, 2018-01-19 at 08:11 +0800, Ming Lei wrote:
>>>>> On Thu, Jan 18, 2018 at 08:37:07AM -0800, Bart Van Assche wrote:
>>>>>> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
>>>>>> index f16096af879a..c59c59cfd2a5 100644
>>>>>> --- a/drivers/md/dm-rq.c
>>>>>> +++ b/drivers/md/dm-rq.c
>>>>>> @@ -761,6 +761,7 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
>>>>>>  		/* Undo dm_start_request() before requeuing */
>>>>>>  		rq_end_stats(md, rq);
>>>>>>  		rq_completed(md, rq_data_dir(rq), false);
>>>>>> +		blk_mq_delay_run_hw_queue(hctx, 100/*ms*/);
>>>>>>  		return BLK_STS_RESOURCE;
>>>>>>  	}
>>>>>>  
>>>>>
>>>>> Nak.
>>>>
>>>> This patch fixes a regression that was introduced by you. You should know
>>>> that regressions are not acceptable. If you don't agree with this patch,
>>>> please fix the root cause.
>>>
>>> Yesterday I sent a patch, did you test that?
>>
>> That patch isn't going to be of much help. It might prevent you from
>> completely stalling, but it might take you tens of seconds to get there.
> 
> Yeah, that is true, and why I marked it as RFC, but the case is so rare to
> happen.

You don't know that. If the resource is very limited, or someone else
is gobbling up all of it, then it may trigger quite often. Granted,
in that case, you need some way of signaling the freeing of those
resources to make it efficient.

>> On top of that, it's a rolling timer that gets added when IO is queued,
>> and re-added if IO is pending when it fires. If the latter case is not
>> true, then it won't get armed again. So if IO fails to queue without
>> any other IO pending, you're pretty much in the same situation as if
> 
> No IO pending detection may take a bit time, we can do it after
> BLK_STS_RESOURCE is returned and SCHED_RESTART is set. I have done
> this way in the following patch, what do you think of it?
> 
> https://github.com/ming1/linux/commit/f866ce0b97b0ae22d033881da7eb07706fd458b4

I think it's overly complicated. As I wrote in a lengthier email earlier
today, just ensure that we have a unique return code from the driver for
when it aborts queuing an IO due to a resource shortage that isn't tied
to it's own consumption of it. When we get that return, do a delayed
queue run after X amount of time to ensure we always try again. If you
want to get fancy (later on), you could track the frequency of such
returns and complain if it's too high.

There's no point in adding a lot of code to check whether we need to run
the queue or not. Just always do it. We won't be doing it more than 100
times per second for the worst case of the condition taking a while to
clear, if we stick to the 10ms re-run time.

-- 
Jens Axboe