Re: [PATCH V4] scsi: core: only re-run queue in scsi_end_request() if device queue is busy
From: Bart Van Assche <bvanassche@acm.org>
Date: 2020-09-02 02:41:00
Also in:
linux-scsi
On 2020-08-17 03:08, Ming Lei wrote:
quoted hunk ↗ jump to hunk
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c index 7c6dd6f75190..a62c29058d26 100644 --- a/drivers/scsi/scsi_lib.c +++ b/drivers/scsi/scsi_lib.c@@ -551,8 +551,27 @@ static void scsi_run_queue_async(struct scsi_device *sdev) if (scsi_target(sdev)->single_lun || !list_empty(&sdev->host->starved_list)) kblockd_schedule_work(&sdev->requeue_work); - else - blk_mq_run_hw_queues(sdev->request_queue, true); + else {
Has this patch been verified with checkpatch? Checkpatch should have warned about the unbalanced braces.
+ /* + * smp_mb() implied in either rq->end_io or blk_mq_free_request + * is for ordering writing .device_busy in scsi_device_unbusy() + * and reading sdev->restarts. + */
Hmm ... I don't see what orders the atomic_dec(&sdev->device_busy) from scsi_device_unbusy() and the atomic_read() below? I don't think that the block layer guarantees ordering of these two memory accesses since both accesses happen in the request completion path.
+ int old = atomic_read(&sdev->restarts);
+
+ if (old) {
+ /*
+ * ->restarts has to be kept as non-zero if there is
+ * new budget contention comes.
There are two verbs in the above sentence ("is" and "comes"). Please remove
"comes" such that the sentence becomes grammatically correct.
+ * + * No need to run queue when either another re-run + * queue wins in updating ->restarts or one new budget + * contention comes. + */ + if (atomic_cmpxchg(&sdev->restarts, old, 0) == old) + blk_mq_run_hw_queues(sdev->request_queue, true); + } + }
Please combine the two if-statements into a single if-statement using "&&" to keep the indentation level low.
quoted hunk ↗ jump to hunk
@@ -1611,8 +1630,34 @@ static void scsi_mq_put_budget(struct request_queue *q) static bool scsi_mq_get_budget(struct request_queue *q) { struct scsi_device *sdev = q->queuedata; + int ret = scsi_dev_queue_ready(q, sdev); + + if (ret) + return true; + + atomic_inc(&sdev->restarts); - return scsi_dev_queue_ready(q, sdev); + /* + * Order writing .restarts and reading .device_busy, and make sure + * .restarts is visible to scsi_end_request(). Its pair is implied by + * __blk_mq_end_request() in scsi_end_request() for ordering + * writing .device_busy in scsi_device_unbusy() and reading .restarts. + * + */ + smp_mb__after_atomic();
Barriers do not guarantee "is visible to". Barriers enforce ordering of memory accesses performed by a certain CPU core. Did you perhaps mean that sdev->restarts must be incremented before the code below reads sdev->device busy?
+ /* + * If all in-flight requests originated from this LUN are completed + * before setting .restarts, 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 request 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; }
Thanks, Bart.