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Subject: Re: [PATCH 1/3] blk-mq: optimise rq sort function
To:     Pavel Begunkov <asml.silence@gmail.com>,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <cover.1574974577.git.asml.silence@gmail.com>
 <a21e91fff1abe201ebbc010d596b034f7b5123de.1574974577.git.asml.silence@gmail.com>
From:   Nikolay Borisov <nborisov@suse.com>
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Date:   Fri, 29 Nov 2019 10:28:31 +0200
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On 28.11.19 г. 23:11 ч., Pavel Begunkov wrote:
> Check "!=" in multi-layer comparisons. The same memory usage, fewer
> instructions, and 2 from 4 jumps are replaced with SETcc.
> 
> Note, that list_sort() doesn't differ 0 and <0.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

My first reaction was this is wrong since you no longer return negative
values. But then I looked into list_sort/merge and this branch
'if (cmp(priv, a, b) <= 0) {' clearly shows this is correct.

So :

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
>  block/blk-mq.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 323c9cb28066..f32a3cfdd34e 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1668,14 +1668,10 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
>  	struct request *rqa = container_of(a, struct request, queuelist);
>  	struct request *rqb = container_of(b, struct request, queuelist);
>  
> -	if (rqa->mq_ctx < rqb->mq_ctx)
> -		return -1;
> -	else if (rqa->mq_ctx > rqb->mq_ctx)
> -		return 1;
> -	else if (rqa->mq_hctx < rqb->mq_hctx)
> -		return -1;
> -	else if (rqa->mq_hctx > rqb->mq_hctx)
> -		return 1;
> +	if (rqa->mq_ctx != rqb->mq_ctx)
> +		return rqa->mq_ctx > rqb->mq_ctx;
> +	if (rqa->mq_hctx != rqb->mq_hctx)
> +		return rqa->mq_hctx > rqb->mq_hctx;
>  
>  	return blk_rq_pos(rqa) > blk_rq_pos(rqb);
>  }
>