linux-kernel - Re: [PATCH 4/4] blk-throttle: cleanup throtl_dequeue

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Date:   Tue, 23 Aug 2022 08:34:10 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com
Subject: Re: [PATCH 4/4] blk-throttle: cleanup throtl_dequeue_tg()

On Tue, Aug 23, 2022 at 09:38:10AM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> Now that throtl_dequeue_tg() is called when the last bio is dispatched,
> there is no need to check the flag THROTL_TG_PENDING, since it's ensured
> to be set when bio is throttled.
> 
> There are no functional changes.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/blk-throttle.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 47142a1dd102..e47506a8ef47 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -570,14 +570,11 @@ static void throtl_enqueue_tg(struct throtl_grp *tg)
>  
>  static void throtl_dequeue_tg(struct throtl_grp *tg)
>  {
> -	if (tg->flags & THROTL_TG_PENDING) {
> -		struct throtl_service_queue *parent_sq =
> -			tg->service_queue.parent_sq;
> +	struct throtl_service_queue *parent_sq = tg->service_queue.parent_sq;
>  
> -		throtl_rb_erase(&tg->rb_node, parent_sq);
> -		--parent_sq->nr_pending;
> -		tg->flags &= ~THROTL_TG_PENDING;
> -	}
> +	throtl_rb_erase(&tg->rb_node, parent_sq);
> +	--parent_sq->nr_pending;
> +	tg->flags &= ~THROTL_TG_PENDING;

Yeah, I don't know about this one. This breaks the symmetry with
throtl_enqueue_tg() and it's a bit odd that we aren't at least
WARN_ON_ONCE() on the flag given what the flag tracks. If you want to do
this, I think the prev approach of just removing the flag is better as that
was symmetric at least.

Thanks.

-- 
tejun