Commit ed5dd6a6 authored by Ming Lei's avatar Ming Lei Committed by Martin K. Petersen
Browse files

scsi: core: Only re-run queue in scsi_end_request() if device queue is busy 

The request queue is currently run unconditionally in scsi_end_request() if
both target queue and host queue are ready.

Recently Long Li reported that cost of a queue run can be very heavy in
case of high queue depth. Improve this situation by only running the
request queue when this LUN is busy.

Link: https://lore.kernel.org/r/20200910075056.36509-1-ming.lei@redhat.com


Reported-by: default avatarLong Li <longli@microsoft.com>
Tested-by: default avatarLong Li <longli@microsoft.com>
Tested-by: default avatarKashyap Desai <kashyap.desai@broadcom.com>
Reviewed-by: default avatarBart Van Assche <bvanassche@acm.org>
Reviewed-by: default avatarHannes Reinecke <hare@suse.de>
Reviewed-by: default avatarEwan D. Milne <emilne@redhat.com>
Reviewed-by: default avatarJohn Garry <john.garry@huawei.com>
Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
Signed-off-by: default avatarMartin K. Petersen <martin.petersen@oracle.com>
parent f97e6e1e

drivers/scsi/scsi_lib.c

+44 −4
Original line number Diff line number Diff line
@@ -549,11 +549,28 @@ static void scsi_mq_uninit_cmd(struct scsi_cmnd *cmd) 
static void scsi_run_queue_async(struct scsi_device *sdev)

{

	if (scsi_target(sdev)->single_lun ||

	    !list_empty(&sdev->host->starved_list))

	    !list_empty(&sdev->host->starved_list)) {

		kblockd_schedule_work(&sdev->requeue_work);

	else

	} else {

		/*

		 * smp_mb() present in sbitmap_queue_clear() or implied in

		 * .end_io is for ordering writing .device_busy in

		 * scsi_device_unbusy() and reading sdev->restarts.

		 */

		int old = atomic_read(&sdev->restarts);



		/*

		 * ->restarts has to be kept as non-zero if new budget

		 *  contention occurs.

		 *

		 *  No need to run queue when either another re-run

		 *  queue wins in updating ->restarts or a new budget

		 *  contention occurs.

		 */

		if (old && atomic_cmpxchg(&sdev->restarts, old, 0) == old)

			blk_mq_run_hw_queues(sdev->request_queue, true);

	}

}



/* Returns false when no more bytes to process, true if there are more */

static bool scsi_end_request(struct request *req, blk_status_t error,
@@ -1612,7 +1629,30 @@ static bool scsi_mq_get_budget(struct request_queue *q) 
{

	struct scsi_device *sdev = q->queuedata;



	return scsi_dev_queue_ready(q, sdev);

	if (scsi_dev_queue_ready(q, sdev))

		return true;



	atomic_inc(&sdev->restarts);



	/*

	 * Orders atomic_inc(&sdev->restarts) and atomic_read(&sdev->device_busy).

	 * .restarts must be incremented before .device_busy is read because the

	 * code in scsi_run_queue_async() depends on the order of these operations.

	 */

	smp_mb__after_atomic();



	/*

	 * If all in-flight requests originated from this LUN are completed

	 * before reading .device_busy, sdev->device_busy will be observed as

	 * zero, then blk_mq_delay_run_hw_queues() will dispatch this request

	 * soon. Otherwise, completion of one of these requests will observe

	 * the .restarts flag, and the request queue will be run for handling

	 * this request, see scsi_end_request().

	 */

	if (unlikely(atomic_read(&sdev->device_busy) == 0 &&

				!scsi_device_blocked(sdev)))

		blk_mq_delay_run_hw_queues(sdev->request_queue, SCSI_QUEUE_DELAY);

	return false;

}



static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,

include/scsi/scsi_device.h

+1 −0
Original line number Diff line number Diff line
@@ -109,6 +109,7 @@ struct scsi_device { 
	atomic_t device_busy;		/* commands actually active on LLDD */

	atomic_t device_blocked;	/* Device returned QUEUE_FULL. */



	atomic_t restarts;

	spinlock_t list_lock;

	struct list_head starved_entry;

	unsigned short queue_depth;	/* How deep of a queue we want */