[PATCH] scsi: core: only re-run queue in scsi_end_request() if device queue is busy

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

Now the requeue queue is run in scsi_end_request() unconditionally if both
target queue and host queue is ready. We should have re-run request queue
only after this device queue becomes busy for restarting this LUN only.

Recently Long Li reported that cost of run queue may be very heavy in
case of high queue depth. So improve this situation by only running
requesut queue when this LUN is busy.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Long Li <longli@microsoft.com>
Cc: linux-block@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/scsi/scsi_lib.c    | 29 +++++++++++++++++++++++++++--
 include/scsi/scsi_device.h |  1 +
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 379533ce8661..212903d5f43c 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -612,7 +612,7 @@ static bool scsi_end_request(struct request *req, blk_status_t error,
 	if (scsi_target(sdev)->single_lun ||
 	    !list_empty(&sdev->host->starved_list))
 		kblockd_schedule_work(&sdev->requeue_work);
-	else
+	else if (READ_ONCE(sdev->restart))
 		blk_mq_run_hw_queues(q, true);
 
 	percpu_ref_put(&q->q_usage_counter);
@@ -1632,8 +1632,33 @@ static bool scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx)
 	struct request_queue *q = hctx->queue;
 	struct scsi_device *sdev = q->queuedata;
 
-	if (scsi_dev_queue_ready(q, sdev))
+	if (scsi_dev_queue_ready(q, sdev)) {
+		WRITE_ONCE(sdev->restart, 0);
 		return true;
+	}
+
+	/*
+	 * If all in-flight requests originated from this LUN are completed
+	 * before setting .restart, sdev->device_busy will be observed as
+	 * zero, then blk_mq_delay_run_hw_queue() will dispatch this request
+	 * soon. Otherwise, completion of one of these request will observe
+	 * the .restart flag, and the request queue will be run for handling
+	 * this request, see scsi_end_request().
+	 *
+	 * However, the .restart flag may be cleared from other dispatch code
+	 * path after one inflight request is completed, then:
+	 *
+	 * 1) if this reqquest is dispatched from scheduler queue or sw queue one
+	 * by one, this request will be handled in that dispatch path too given
+	 * the request still stays at scheduler/sw queue when calling .get_budget()
+	 * callback.
+	 *
+	 * 2) if this request is dispatched from hctx->dispatch or
+	 * blk_mq_flush_busy_ctxs(), this request will be put into hctx->dispatch
+	 * list soon, and blk-mq will be responsible for covering it, see
+	 * blk_mq_dispatch_rq_list().
+	 */
+	WRITE_ONCE(sdev->restart, 1);
 
 	if (atomic_read(&sdev->device_busy) == 0 && !scsi_device_blocked(sdev))
 		blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY);
diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index 202f4d6a4342..9d8ca662ae86 100644
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -109,6 +109,7 @@ struct scsi_device {
 	atomic_t device_busy;		/* commands actually active on LLDD */
 	atomic_t device_blocked;	/* Device returned QUEUE_FULL. */
 
+	unsigned int restart;
 	spinlock_t list_lock;
 	struct list_head cmd_list;	/* queue of in use SCSI Command structures */
 	struct list_head starved_entry;
-- 
2.20.1