linux-kernel - Re: [PATCH -next 2/2] block, bfq: make bfq_has

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Date:   Wed, 11 May 2022 16:08:32 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     jack@...e.cz, paolo.valente@...aro.org, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next 2/2] block, bfq: make bfq_has_work() more accurate

On Tue 10-05-22 21:16:29, Yu Kuai wrote:
> bfq_has_work() is using busy_queues currently, which is not accurate
> because bfq_queue is busy doesn't represent that it has requests. Since
> bfqd aready has a counter 'queued' to record how many requests are in
> bfq, use it instead of busy_queues.
> 
> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
> lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

So did you find this causing any real problem? Because bfq queue is
accounted among busy queues once bfq_add_bfqq_busy() is called. And that
happens once a new request is inserted into the queue so it should be very
similar to bfqd->queued.

								Honza

> ---
>  block/bfq-iosched.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 61750696e87f..1d2f8110c26b 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
>  	struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
>  
>  	/*
> -	 * Avoiding lock: a race on bfqd->busy_queues should cause at
> +	 * Avoiding lock: a race on bfqd->queued should cause at
>  	 * most a call to dispatch for nothing
>  	 */
>  	return !list_empty_careful(&bfqd->dispatch) ||
> -		bfq_tot_busy_queues(bfqd) > 0;
> +		READ_ONCE(bfqd->queued);
>  }
>  
>  static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR