linux-kernel - Re: [PATCH 2/4] block, bfq: avoid moving bfqq to it's parent bfqg

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Date:   Tue, 21 Dec 2021 11:16:59 +0100
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, paolo.valente@...aro.org,
        jack@...e.cz, fchecconi@...il.com, avanzini.arianna@...il.com,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH 2/4] block, bfq: avoid moving bfqq to it's parent bfqg

On Tue 21-12-21 11:21:33, Yu Kuai wrote:
> Moving bfqq to it's parent bfqg is pointless.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

Did you notice that this is happening often enough that the check is worth
it? Where do we do this?

								Honza

> ---
>  block/bfq-cgroup.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 24a5c5329bcd..0f62546a72d4 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -645,6 +645,11 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  		   struct bfq_group *bfqg)
>  {
>  	struct bfq_entity *entity = &bfqq->entity;
> +	struct bfq_group *old_parent = bfq_group(bfqq);
> +
> +	/* No point to move bfqq to the same group */
> +	if (old_parent == bfqg)
> +		return;
>  
>  	/*
>  	 * Get extra reference to prevent bfqq from being freed in
> @@ -666,7 +671,7 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  		bfq_deactivate_bfqq(bfqd, bfqq, false, false);
>  	else if (entity->on_st_or_in_serv)
>  		bfq_put_idle_entity(bfq_entity_service_tree(entity), entity);
> -	bfqg_and_blkg_put(bfqq_group(bfqq));
> +	bfqg_and_blkg_put(old_parent);
>  
>  	if (entity->parent &&
>  	    entity->parent->last_bfqq_created == bfqq)
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR