On 4/8/22 00:39, Yu Kuai wrote:
bt_wait_ptr() will increase 'wait_index', however, if blk_mq_get_tag()
get a tag successfully after bt_wait_ptr() is called and before
sbitmap_prepare_to_wait() is called, then the 'ws' is skipped. This
behavior might cause 8 waitqueues to be unbalanced.
Move bt_wait_ptr() later should reduce the problem when the disk is
under high io preesure.
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
block/blk-mq-tag.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 68ac23d0b640..228a0001694f 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -155,7 +155,6 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
if (data->flags & BLK_MQ_REQ_NOWAIT)
return BLK_MQ_NO_TAG;
- ws = bt_wait_ptr(bt, data->hctx);
do {
struct sbitmap_queue *bt_prev;
@@ -174,6 +173,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
if (tag != BLK_MQ_NO_TAG)
break;
+ ws = bt_wait_ptr(bt, data->hctx);
sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
tag = __blk_mq_get_tag(data, bt);
@@ -201,8 +201,6 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
*/
if (bt != bt_prev)
sbitmap_queue_wake_up(bt_prev);
-
- ws = bt_wait_ptr(bt, data->hctx);
} while (1);
Is it necessary to call bt_wait_ptr() during every loop iteration or
only if bt != bt_prev? Would calling bt_wait_ptr() only if bt != bt_prev
help to reduce unfairness further?
Thanks,
Bart.
