Re: [PATCH 04/14] blk-mq-sched: improve dispatching from sw queue

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

On Mon, Jul 31, 2017 at 11:34:35PM +0000, Bart Van Assche wrote:
> On Tue, 2017-08-01 at 00:51 +0800, Ming Lei wrote:
> > SCSI devices use host-wide tagset, and the shared
> > driver tag space is often quite big. Meantime
> > there is also queue depth for each lun(.cmd_per_lun),
> > which is often small.
> > 
> > So lots of requests may stay in sw queue, and we
> > always flush all belonging to same hw queue and
> > dispatch them all to driver, unfortunately it is
> > easy to cause queue busy becasue of the small
> > per-lun queue depth. Once these requests are flushed
> > out, they have to stay in hctx->dispatch, and no bio
> > merge can participate into these requests, and
> > sequential IO performance is hurted.
> > 
> > This patch improves dispatching from sw queue when
> > there is per-request-queue queue depth by taking
> > request one by one from sw queue, just like the way
> > of IO scheduler.
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq-sched.c | 25 +++++++++++++++----------
> >  1 file changed, 15 insertions(+), 10 deletions(-)
> > 
> > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > index 47a25333a136..3510c01cb17b 100644
> > --- a/block/blk-mq-sched.c
> > +++ b/block/blk-mq-sched.c
> > @@ -96,6 +96,9 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> >  	const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
> >  	bool can_go = true;
> >  	LIST_HEAD(rq_list);
> > +	struct request *(*dispatch_fn)(struct blk_mq_hw_ctx *) =
> > +		has_sched_dispatch ? e->type->ops.mq.dispatch_request :
> > +			blk_mq_dispatch_rq_from_ctxs;
> >  
> >  	/* RCU or SRCU read lock is needed before checking quiesced flag */
> >  	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
> > @@ -126,26 +129,28 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> >  	if (!list_empty(&rq_list)) {
> >  		blk_mq_sched_mark_restart_hctx(hctx);
> >  		can_go = blk_mq_dispatch_rq_list(q, &rq_list);
> > -	} else if (!has_sched_dispatch) {
> > +	} else if (!has_sched_dispatch && !q->queue_depth) {
> >  		blk_mq_flush_busy_ctxs(hctx, &rq_list);
> >  		blk_mq_dispatch_rq_list(q, &rq_list);
> > +		can_go = false;
> >  	}
> >  
> > +	if (!can_go)
> > +		return;
> > +
> >  	/*
> >  	 * We want to dispatch from the scheduler if we had no work left
> >  	 * on the dispatch list, OR if we did have work but weren't able
> >  	 * to make progress.
> >  	 */
> > -	if (can_go && has_sched_dispatch) {
> > -		do {
> > -			struct request *rq;
> > +	do {
> > +		struct request *rq;
> >  
> > -			rq = e->type->ops.mq.dispatch_request(hctx);
> > -			if (!rq)
> > -				break;
> > -			list_add(&rq->queuelist, &rq_list);
> > -		} while (blk_mq_dispatch_rq_list(q, &rq_list));
> > -	}
> > +		rq = dispatch_fn(hctx);
> > +		if (!rq)
> > +			break;
> > +		list_add(&rq->queuelist, &rq_list);
> > +	} while (blk_mq_dispatch_rq_list(q, &rq_list));
> >  }
> 
> Hello Ming,
> 
> Although I like the idea behind this patch, I'm afraid that this patch will
> cause a performance regression for high-performance SCSI LLD drivers, e.g.
> ib_srp. Have you considered to rework this patch as follows:
> * Remove the code under "else if (!has_sched_dispatch && !q->queue_depth) {".

This will affect devices such as NVMe in which busy isn't triggered
basically, so better to not do this.

> * Modify all blk_mq_dispatch_rq_list() functions such that these dispatch up
>   to cmd_per_lun - (number of requests in progress) at once.

How can we get the accurate 'number of requests in progress' efficiently?

And we have done it in this way for blk-mq scheduler already, so it
shouldn't be a problem.

>From my test data of mq-deadline on lpfc, the performance is good,
please see it in cover letter.


Thanks,
Ming