blk-mq: dequeue request one by one from sw queue if hctx is busy

It won't be efficient to dequeue request one by one from sw queue,
but we have to do that when queue is busy for better merge performance.

This patch takes the Exponential Weighted Moving Average(EWMA) to figure
out if queue is busy, then only dequeue request one by one from sw queue
when queue is busy.

Fixes: b347689ffbca ("blk-mq-sched: improve dispatching from sw queue")
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Reported-by: Kashyap Desai <kashyap.desai@broadcom.com>
Tested-by: Kashyap Desai <kashyap.desai@broadcom.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 2 insertions, 9 deletions

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index f3b4b5ceb4d1..fdc129e64cc4 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -206,15 +206,8 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
                 }
         } else if (has_sched_dispatch) {
                 blk_mq_do_dispatch_sched(hctx);
-        } else if (q->mq_ops->get_budget) {
-                /*
-                 * If we need to get budget before queuing request, we
-                 * dequeue request one by one from sw queue for avoiding
-                 * to mess up I/O merge when dispatch runs out of resource.
-                 *
-                 * TODO: get more budgets, and dequeue more requests in
-                 * one time.
-                 */
+        } else if (hctx->dispatch_busy) {
+                /* dequeue request one by one from sw queue if queue is busy */
                 blk_mq_do_dispatch_ctx(hctx);
         } else {
                 blk_mq_flush_busy_ctxs(hctx, &rq_list);