Messages in this thread | | | Date | Fri, 8 Apr 2022 07:20:41 -0700 | Subject | Re: [PATCH -next RFC v2 2/8] blk-mq: call 'bt_wait_ptr()' later in blk_mq_get_tag() | From | Bart Van Assche <> |
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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@huawei.com> > --- > 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.
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