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

Bart Van Assche
2017-07-31T23:35:26Z

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.
>=20
> 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.
>=20
> 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.
>=20
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-mq-sched.c | 25 +++++++++++++++----------
> 1 file changed, 15 insertions(+), 10 deletions(-)
>=20
> 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_ct=
x *hctx)
> const bool has_sched_dispatch =3D e && e->type->ops.mq.dispatch_request=
;
> bool can_go =3D true;
> LIST_HEAD(rq_list);
> + struct request *(*dispatch_fn)(struct blk_mq_hw_ctx *) =3D
> + has_sched_dispatch ? e->type->ops.mq.dispatch_request :
> + blk_mq_dispatch_rq_from_ctxs;
> =20
> /* 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_h=
w_ctx *hctx)
> if (!list_empty(&rq_list)) {
> blk_mq_sched_mark_restart_hctx(hctx);
> can_go =3D 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 =3D false;
> }
> =20
> + 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;
> =20
> - rq =3D 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 =3D 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) {=
".
* Modify all blk_mq_dispatch_rq_list() functions such that these dispatch u=
p
to cmd_per_lun - (number of requests in progress) at once.

Thanks,

Bart.=