linux-kernel - Re: [RFC PATCH] blk-cgroup: prevent rcu_sched detected stalls warnings in blkg_destroy

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Date:   Wed, 25 Nov 2020 07:32:48 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, zhangxiaoxu5@...wei.com, houtao1@...wei.com
Subject: Re: [RFC PATCH] blk-cgroup: prevent rcu_sched detected stalls
 warnings in blkg_destroy_all()

Hello,

Thanks for the fix. A couple comments below.

On Sat, Nov 21, 2020 at 04:34:20PM +0800, Yu Kuai wrote:
> +#define BLKG_DESTROY_BATH 4096

I think you meant BLKG_DESTROY_BATCH.

>  static void blkg_destroy_all(struct request_queue *q)
>  {
>  	struct blkcg_gq *blkg, *n;
> +	int count = BLKG_DESTROY_BATH;

But might as well just write 4096 here.

>  	spin_lock_irq(&q->queue_lock);
>  	list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
>  		struct blkcg *blkcg = blkg->blkcg;
>  
> +		/*
> +		 * If the list is too long, the loop can took a long time,
> +		 * thus relese the lock for a while when a batch of blkcg
> +		 * were destroyed.
> +		 */
> +		if (!(--count)) {
> +			count = BLKG_DESTROY_BATH;
> +			spin_unlock_irq(&q->queue_lock);
> +			cond_resched();
> +			spin_lock_irq(&q->queue_lock);

You can't continue iteration after dropping both locks. You'd have to jump
out of loop and start list_for_each_entry_safe() again.

> +		}
>  		spin_lock(&blkcg->lock);
>  		blkg_destroy(blkg);
>  		spin_unlock(&blkcg->lock);

Thanks.

-- 
tejun