On Sat, Aug 05, 2017 at 02:56:46PM +0800, Ming Lei wrote:
> When hw queue is busy, we shouldn't take requests from
> scheduler queue any more, otherwise IO merge will be
> difficult to do.
>
> This patch fixes the awful IO performance on some
> SCSI devices(lpfc, qla2xxx, ...) when mq-deadline/kyber
> is used by not taking requests if hw queue is busy.
Jens added this behavior in 64765a75ef25 ("blk-mq-sched: ask scheduler
for work, if we failed dispatching leftovers"). That change was a big
performance improvement, but we didn't figure out why. We'll need to dig
up whatever test Jens was doing to make sure it doesn't regress.
> Reviewed-by: Bart Van Assche <bart.vanassche@xxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> block/blk-mq-sched.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 4ab69435708c..845e5baf8af1 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -94,7 +94,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> struct request_queue *q = hctx->queue;
> struct elevator_queue *e = q->elevator;
> const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
> - bool did_work = false;
> + bool do_sched_dispatch = true;
> LIST_HEAD(rq_list);
>
> /* RCU or SRCU read lock is needed before checking quiesced flag */
> @@ -125,7 +125,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> */
> if (!list_empty(&rq_list)) {
> blk_mq_sched_mark_restart_hctx(hctx);
> - did_work = blk_mq_dispatch_rq_list(q, &rq_list);
> + do_sched_dispatch = blk_mq_dispatch_rq_list(q, &rq_list);
> } else if (!has_sched_dispatch) {
> blk_mq_flush_busy_ctxs(hctx, &rq_list);
> blk_mq_dispatch_rq_list(q, &rq_list);
> @@ -136,7 +136,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> * on the dispatch list, OR if we did have work but weren't able
> * to make progress.
> */
> - if (!did_work && has_sched_dispatch) {
> + if (do_sched_dispatch && has_sched_dispatch) {
> do {
> struct request *rq;
>
> --
> 2.9.4
>
