Hello Kuai.
On Thu, Dec 02, 2021 at 09:04:40PM +0800, Yu Kuai <yukuai3@xxxxxxxxxx> 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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