linux-kernel - Re: [PATCH v4 2/2] block: cancel all throttled bios in del

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Date:   Thu, 2 Dec 2021 15:48:18 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     hch@...radead.org, tj@...nel.org, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH v4 2/2] block: cancel all throttled bios in del_gendisk()

Hello Kuai.

On Thu, Dec 02, 2021 at 09:04:40PM +0800, Yu Kuai <yukuai3@...wei.com> wrote:
> For example, if user thread is throttled with low bps while it's
> issuing large io, and the device is deleted. The user thread will
> wait for a long time for io to return.

Do I understand correctly the "long time" here is
outstanding_IO_size/throttled_bandwidth? Or are you getting at some
other cause/longer time?

> +void blk_throtl_cancel_bios(struct request_queue *q)
> +{
> +	struct throtl_data *td = q->td;
> +	struct bio_list bio_list_on_stack;
> +	struct blkcg_gq *blkg;
> +	struct cgroup_subsys_state *pos_css;
> +	struct bio *bio;
> +	int rw;
> +
> +	bio_list_init(&bio_list_on_stack);
> +
> +	/*
> +	 * hold queue_lock to prevent concurrent with dispatching
> +	 * throttled bios by timer.
> +	 */
> +	spin_lock_irq(&q->queue_lock);

You've replaced the rcu_read_lock() with the queue lock but...

> +
> +	/*
> +	 * Drain each tg while doing post-order walk on the blkg tree, so
> +	 * that all bios are propagated to td->service_queue.  It'd be
> +	 * better to walk service_queue tree directly but blkg walk is
> +	 * easier.
> +	 */
> +	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg)
> +		tg_drain_bios(&blkg_to_tg(blkg)->service_queue);

...you also need the rcu_read_lock() here since you may encounter a
(descendant) blkcg that's removed concurrently.

(I may miss some consequences of doing this under the queue_lock so if
the concurrent removal is ruled out, please make a comment about it.)


Regards,
Michal

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