linux-kernel - Re: [PATCH RESEND v2 01/10] block, bfq: correctly raise inject limit in bfq_choose_bfqq_for

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Message-ID: <20230102151329.xzjalwcphmupf4jm@quack3>
Date:   Mon, 2 Jan 2023 16:13:29 +0100
From:   Jan Kara <jack@...e.cz>
To:     Kemeng Shi <shikemeng@...weicloud.com>
Cc:     paolo.valente@...aro.org, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        jack@...e.cz, hch@....de, damien.lemoal@....com
Subject: Re: [PATCH RESEND v2 01/10] block, bfq: correctly raise inject limit
 in bfq_choose_bfqq_for_injection

On Fri 23-12-22 03:16:32, Kemeng Shi wrote:
> Function bfq_choose_bfqq_for_injection may temporarily raise inject limit
> to one request if current inject_limit is 0 before search of the source
> queue for injection. However the search below will reset inject limit to
> bfqd->in_service_queue which is zero for raised inject limit. Then the
> temporarily raised inject limit never works as expected.
> Assigment limit to bfqd->in_service_queue in search is needed as limit
> maybe overwriten to min_t(unsigned int, 1, limit) for condition that
> a large in-flight request is on non-rotational devices in found queue.
> So we need to reset limit to bfqd->in_service_queue for normal case.
> 
> Actually, we have already make sure bfqd->rq_in_driver is < limit before
> search, then
>  -Limit is >= 1 as bfqd->rq_in_driver is >= 0. Then min_t(unsigned int,
> 1, limit) is always 1. So we can simply check bfqd->rq_in_driver with
> 1 instead of result of min_t(unsigned int, 1, limit) for larget request in
> non-rotational device case to avoid overwritting limit and the bug is gone.
>  -For normal case, we have already check bfqd->rq_in_driver is < limit,
> so we can return found bfqq unconditionally to remove unncessary check.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>

The fix looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  block/bfq-iosched.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index a72304c728fc..1220c41fc767 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -4641,12 +4641,10 @@ bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
>  			 */
>  			if (blk_queue_nonrot(bfqd->queue) &&
>  			    blk_rq_sectors(bfqq->next_rq) >=
> -			    BFQQ_SECT_THR_NONROT)
> -				limit = min_t(unsigned int, 1, limit);
> -			else
> -				limit = in_serv_bfqq->inject_limit;
> -
> -			if (bfqd->rq_in_driver < limit) {
> +			    BFQQ_SECT_THR_NONROT &&
> +			    bfqd->rq_in_driver >= 1)
> +				continue;
> +			else {
>  				bfqd->rqs_injected = true;
>  				return bfqq;
>  			}
> -- 
> 2.30.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR