[PATCH v3 3/5] blk-mq: Clarify comments in blk_mq_dispatch_rq_list()

Bart Van Assche
2017-04-06T18:10:48Z

The blk_mq_dispatch_rq_list() implementation got modified several
times but the comments in that function were not updated every
time. Since it is nontrivial what is going on, update the comments
in blk_mq_dispatch_rq_list().

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
---
block/blk-mq.c | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index dc83aec338d9..4db48ad76878 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1063,8 +1063,8 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list)
*/
if (!list_empty(list)) {
/*
- * If we got a driver tag for the next request already,
- * free it again.
+ * If an I/O scheduler has been configured and we got a driver
+ * tag for the next request already, free it again.
*/
rq = list_first_entry(list, struct request, queuelist);
blk_mq_put_driver_tag(rq);
@@ -1074,16 +1074,24 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list)
spin_unlock(&hctx->lock);

/*
- * the queue is expected stopped with BLK_MQ_RQ_QUEUE_BUSY, but
- * it's possible the queue is stopped and restarted again
- * before this. Queue restart will dispatch requests. And since
- * requests in rq_list aren't added into hctx->dispatch yet,
- * the requests in rq_list might get lost.
+ * If SCHED_RESTART was set by the caller of this function and
+ * it is no longer set that means that it was cleared by another
+ * thread and hence that a queue rerun is needed.
*
- * blk_mq_run_hw_queue() already checks the STOPPED bit
+ * If TAG_WAITING is set that means that an I/O scheduler has
+ * been configured and another thread is waiting for a driver
+ * tag. To guarantee fairness, do not rerun this hardware queue
+ * but let the other thread grab the driver tag.
*
- * If RESTART or TAG_WAITING is set, then let completion restart
- * the queue instead of potentially looping here.
+ * If no I/O scheduler has been configured it is possible that
+ * the hardware queue got stopped and restarted before requests
+ * were pushed back onto the dispatch list. Rerun the queue to
+ * avoid starvation. Notes:
+ * - blk_mq_run_hw_queue() checks whether or not a queue has
+ * been stopped before rerunning a queue.
+ * - Some but not all block drivers stop a queue before
+ * returning BLK_MQ_RQ_QUEUE_BUSY. Two exceptions are scsi-mq
+ * and dm-rq.
*/
if (!blk_mq_sched_needs_restart(hctx) &&
!test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))
--
2.12.0