linux-kernel - Re: [PATCH -next] block, bfq: cleanup 'bfqg->online'

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Message-ID: <20230201131037.6frw2kpc54k4sx7a@quack3>
Date:   Wed, 1 Feb 2023 14:10:37 +0100
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     jack@...e.cz, tj@...nel.org, josef@...icpanda.com, axboe@...nel.dk,
        paolo.valente@...aro.org, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next] block, bfq: cleanup 'bfqg->online'

On Wed 01-02-23 20:06:09, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> After commit dfd6200a0954 ("blk-cgroup: support to track if policy is
> online"), there is no need to do this again in bfq.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

So I agree this is nice to do but it isn't so simple. BFQ relies on the
fact that 'online' is cleared under bfqd->lock so we cannot associate bio
in bfq_bio_bfqg() with a bfqg that has already its bfq_pd_offline()
function run.

Maybe if you set 'online' to false before calling ->pd_offline() things
would work fine for BFQ.

								Honza
> ---
>  block/bfq-cgroup.c  | 4 +---
>  block/bfq-iosched.h | 2 --
>  2 files changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index b42956ab5550..a35136dae713 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -551,7 +551,6 @@ static void bfq_pd_init(struct blkg_policy_data *pd)
>  	bfqg->bfqd = bfqd;
>  	bfqg->active_entities = 0;
>  	bfqg->num_queues_with_pending_reqs = 0;
> -	bfqg->online = true;
>  	bfqg->rq_pos_tree = RB_ROOT;
>  }
>  
> @@ -614,7 +613,7 @@ struct bfq_group *bfq_bio_bfqg(struct bfq_data *bfqd, struct bio *bio)
>  			continue;
>  		}
>  		bfqg = blkg_to_bfqg(blkg);
> -		if (bfqg->online) {
> +		if (bfqg->pd.online) {
>  			bio_associate_blkg_from_css(bio, &blkg->blkcg->css);
>  			return bfqg;
>  		}
> @@ -985,7 +984,6 @@ static void bfq_pd_offline(struct blkg_policy_data *pd)
>  
>  put_async_queues:
>  	bfq_put_async_queues(bfqd, bfqg);
> -	bfqg->online = false;
>  
>  	spin_unlock_irqrestore(&bfqd->lock, flags);
>  	/*
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 75cc6a324267..69aaee52285a 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -1009,8 +1009,6 @@ struct bfq_group {
>  
>  	/* reference counter (see comments in bfq_bic_update_cgroup) */
>  	refcount_t ref;
> -	/* Is bfq_group still online? */
> -	bool online;
>  
>  	struct bfq_entity entity;
>  	struct bfq_sched_data sched_data;
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR