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

[Ming Lei]

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.

> 
> 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

-- 
Ming