Re: [PATCH v4 3/6] blk-mq: Clarify comments in blk_mq_dispatch_rq_list()

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig 

[PATCH v4 3/6] blk-mq: Clarify comments in blk_mq_dispatch_rq_list()

[Bart Van Assche]

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 
Cc: Omar Sandoval 
Cc: Christoph Hellwig 
Cc: Hannes Reinecke 
---
 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 dba34eb79a08..aff85d41cea3 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(>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, >state))
-- 
2.12.0