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

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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: Long Li <longli@microsoft.com>
Tested-by: Long Li <longli@microsoft.com>
Tested-by: Kashyap Desai <kashyap.desai@broadcom.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
This commit is contained in:
Ming Lei 2020-09-10 15:50:56 +08:00 committed by Martin K. Petersen
parent f97e6e1eab
commit ed5dd6a67d
2 changed files with 45 additions and 4 deletions
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48
drivers/scsi/scsi_lib.c
View file

@ -549,10 +549,27 @@ 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
blk_mq_run_hw_queues(sdev->request_queue, true);
} 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 */
@ -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,

1
include/scsi/scsi_device.h
View file

@ -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 */