Re: [PATCH] dm-mpath: return DM_MAPIO_REQUEUE in case of rq allocation failure

[Ming Lei <ming.lei@redhat.com>]

On Fri, Sep 22, 2017 at 03:06:16PM +0000, Bart Van Assche wrote:
> On Fri, 2017-09-22 at 09:35 +0800, Ming Lei wrote:
> > +		/*
> > +		 * blk-mq's SCHED_RESTART can cover this requeue, so
> > +		 * we needn't to deal with it by DELAY_REQUEUE. More
> > +		 * importantly, we have to return DM_MAPIO_REQUEUE
> > +		 * so that blk-mq can get the queue busy feedback,
> > +		 * otherwise I/O merge can be hurt.
> > +		 */
> > +		if (q->mq_ops)
> > +			return DM_MAPIO_REQUEUE;
> > +		else
> > +			return DM_MAPIO_DELAY_REQUEUE;
> >  	}
> 
> This patch is inferior to what I posted because this patch does not avoid
> the delay if multiple LUNs are associated with the same SCSI host. Consider
> e.g. the following configuration:
> * A single SCSI host with two SCSI LUNs associated to that host, e.g. /dev/sda
>   and /dev/sdb.
> * A dm-mpath instance has been created on top of /dev/sda.
> If all tags are in use by requests queued to /dev/sdb, no dm requests are in
> progress and a request is submitted against the dm-mpath device then the
> blk_get_request(q, GFP_ATOMIC) call will fail. The request will be requeued
> and the queue will be rerun after a delay.
> 
> My patch does not introduce a delay in this case.

That delay may not matter because SCHED_RESTART will run queue just
after one request is completed.

There is at least one issue with get_request(GFP_NOIO): AIO
performance regression may not be caused, or even AIO may not
be possible. For example, user runs fio(libaio, randread, single
job, queue depth: 64, device: dm-mpath disk), if get_request(GFP_NOIO)
often blocks because of shared tags or out of tag, the actual queue
depth won't reach 64 at all, and may be just 1 in the worst case.
Once the actual queue depth is decreased much, random I/O performance
should be hurt a lot.

-- 
Ming