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

Ming Lei
2017-08-01T10:50:15Z

On Tue, Aug 01, 2017 at 06:17:18PM +0800, Ming Lei wrote:
> 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.

Forget to mention, ctx->list is one per-cpu list and the lock is percpu
lock, so changing to this way shouldn't be a performance issue.

--
Ming