linux-kernel - Re: [PATCH -next 4/5] blk-iocost: bypass if only one cgroup issues io

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Date:   Tue, 11 Oct 2022 07:02:30 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next 4/5] blk-iocost: bypass if only one cgroup issues io

On Tue, Oct 11, 2022 at 04:35:46PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> In this special case, there is no need to throttle io.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/blk-iocost.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 5acc5f13bbd6..32e7e416d67c 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -2564,8 +2564,13 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
>  	bool use_debt, ioc_locked;
>  	unsigned long flags;
>  
> -	/* bypass IOs if disabled, still initializing, or for root cgroup */
> -	if (!ioc->enabled || !iocg || !iocg->level)
> +	/*
> +	 * bypass IOs if disabled, still initializing, for root cgroup,
> +	 * or the cgroup is the only cgroup with io.
> +	 */
> +	if (!ioc->enabled || !iocg || !iocg->level ||
> +	    (iocg->hweight_inuse == WEIGHT_ONE &&
> +	     atomic_read(&ioc->hweight_gen) == iocg->hweight_gen))

I'm not sure about this one. Bypassing here means that we lose track of how
much IO it's issuing which can affect future throttling decisions, right?

Thanks.

-- 
tejun