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SubjectRe: [PATCH -next RFC v2 2/8] blk-mq: call 'bt_wait_ptr()' later in blk_mq_get_tag()
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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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