linux-kernel - Re: [PATCH 2/3] blk-mq: Eliminate superfluous check of BLK_MQ_S_TAG

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Date:	Wed, 03 Sep 2014 14:40:14 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Alexander Gordeev <agordeev@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] blk-mq: Eliminate superfluous check of BLK_MQ_S_TAG_ACTIVE
 flag

On 09/03/2014 02:33 PM, Alexander Gordeev wrote:
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> Cc: Jens Axboe <axboe@...nel.dk>
> ---
>  block/blk-mq-tag.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 4953b64..d1eb579 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -60,8 +60,7 @@ static inline void bt_index_atomic_inc(atomic_t *index)
>   */
>  void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
>  {
> -	if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
> -	    !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
> +	if (!test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
>  		atomic_inc(&hctx->tags->active_queues);

That's done on purpose as well, it's a lot faster to do the (non-atomic)
check first and only do the expensive test and set if needed.

See 7fcbbaf18392 for similar construct and actual numbers on a real case.

-- 
Jens Axboe

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