Re: [PATCH 2/6] block, bfq: don't grab queue_lock from io path

[Damien Le Moal <dlemoal@kernel.org>]

On 7/22/25 4:24 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> Currently issue io can grab queue_lock three times from bfq_bio_merge(),
> bfq_limit_depth() and bfq_prepare_request(), the queue_lock is not
> necessary if icq is already created:
> 
> - queue_usage_counter is already grabbed and queue won't exist;
> - current thread won't exist;
> - if other thread is allocating and inserting new icq to ioc->icq_tree,
>   rcu can be used to protect lookup icq from the raidx tree, it's safe
>   to use extracted icq until queue or current thread exit;
> 
> If ioc or icq is not created, then bfq_prepare_request() will create it,
> which means the task is issuing io to queue the first time, this can
> consider a slow path and queue_lock will still be held to protect
> inserting allocated icq to ioc->icq_tree.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/bfq-iosched.c | 24 +++++++-----------------
>  block/blk-ioc.c     | 43 ++++++++++++++++++++++++++++++++++++++-----
>  block/blk.h         |  2 +-
>  3 files changed, 46 insertions(+), 23 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 0cb1e9873aab..58d57c482acd 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -454,17 +454,13 @@ static struct bfq_io_cq *icq_to_bic(struct io_cq *icq)
>   */
>  static struct bfq_io_cq *bfq_bic_lookup(struct request_queue *q)
>  {
> -	struct bfq_io_cq *icq;
> -	unsigned long flags;
> -
> -	if (!current->io_context)
> -		return NULL;
> +	struct io_cq *icq;
>  
> -	spin_lock_irqsave(&q->queue_lock, flags);
> -	icq = icq_to_bic(ioc_lookup_icq(q));
> -	spin_unlock_irqrestore(&q->queue_lock, flags);
> +	rcu_read_lock();
> +	icq = ioc_lookup_icq_rcu(q);
> +	rcu_read_unlock();
>  
> -	return icq;
> +	return icq_to_bic(icq);

icq cannot be NULL here ? If it can, that needs checking, otherwise,
icq_to_bic() will return a bad address.

>  }
>  
>  /*
> @@ -2456,16 +2452,10 @@ static void bfq_remove_request(struct request_queue *q,
>  static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
>  		unsigned int nr_segs)
>  {
> +	/* bic will not be freed until current or elevator exit */

I would drop this comment, or move it somewhere else as having a comment in the
declarations seems odd.

> +	struct bfq_io_cq *bic = bfq_bic_lookup(q);
>  	struct bfq_data *bfqd = q->elevator->elevator_data;
>  	struct request *free = NULL;
> -	/*
> -	 * bfq_bic_lookup grabs the queue_lock: invoke it now and
> -	 * store its return value for later use, to avoid nesting
> -	 * queue_lock inside the bfqd->lock. We assume that the bic
> -	 * returned by bfq_bic_lookup does not go away before
> -	 * bfqd->lock is taken.
> -	 */
> -	struct bfq_io_cq *bic = bfq_bic_lookup(q);
>  	bool ret;
>  
>  	spin_lock_irq(&bfqd->lock);
> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> index ce82770c72ab..0be097a37e22 100644
> --- a/block/blk-ioc.c
> +++ b/block/blk-ioc.c
> @@ -314,7 +314,7 @@ int __copy_io(unsigned long clone_flags, struct task_struct *tsk)
>   * Look up io_cq associated with @ioc - @q pair from @ioc.  Must be called
>   * with @q->queue_lock held.
>   */
> -struct io_cq *ioc_lookup_icq(struct request_queue *q)
> +static struct io_cq *ioc_lookup_icq(struct request_queue *q)
>  {
>  	struct io_context *ioc = current->io_context;
>  	struct io_cq *icq;
> @@ -341,7 +341,40 @@ struct io_cq *ioc_lookup_icq(struct request_queue *q)
>  	rcu_read_unlock();
>  	return icq;
>  }
> -EXPORT_SYMBOL(ioc_lookup_icq);
> +
> +/**
> + * ioc_lookup_icq_rcu - lookup io_cq from ioc in io path
> + * @q: the associated request_queue
> + *
> + * Look up io_cq associated with @ioc - @q pair from @ioc.  Must be called
> + * from io path, either return NULL if current issue io to @q for the first
> + * time, or return a valid icq.
> + */
> +struct io_cq *ioc_lookup_icq_rcu(struct request_queue *q)
> +{
> +	struct io_context *ioc = current->io_context;
> +	struct io_cq *icq;
> +
> +	WARN_ON_ONCE(percpu_ref_is_zero(&q->q_usage_counter));
> +
> +	if (!ioc)
> +		return NULL;
> +
> +	icq = rcu_dereference(ioc->icq_hint);
> +	if (icq && icq->q == q)
> +		return icq;
> +
> +	icq = radix_tree_lookup(&ioc->icq_tree, q->id);
> +	if (!icq)
> +		return NULL;
> +
> +	if (WARN_ON_ONCE(icq->q != q))
> +		return NULL;
> +
> +	rcu_assign_pointer(ioc->icq_hint, icq);
> +	return icq;
> +}
> +EXPORT_SYMBOL(ioc_lookup_icq_rcu);
>  
>  /**
>   * ioc_create_icq - create and link io_cq
> @@ -420,9 +453,9 @@ struct io_cq *ioc_find_get_icq(struct request_queue *q)
>  	} else {
>  		get_io_context(ioc);
>  
> -		spin_lock_irq(&q->queue_lock);
> -		icq = ioc_lookup_icq(q);
> -		spin_unlock_irq(&q->queue_lock);
> +		rcu_read_lock();
> +		icq = ioc_lookup_icq_rcu(q);
> +		rcu_read_unlock();
>  	}
>  
>  	if (!icq) {
> diff --git a/block/blk.h b/block/blk.h
> index 468aa83c5a22..3c078e517d59 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -460,7 +460,7 @@ static inline void req_set_nomerge(struct request_queue *q, struct request *req)
>   * Internal io_context interface
>   */
>  struct io_cq *ioc_find_get_icq(struct request_queue *q);
> -struct io_cq *ioc_lookup_icq(struct request_queue *q);
> +struct io_cq *ioc_lookup_icq_rcu(struct request_queue *q);
>  #ifdef CONFIG_BLK_ICQ
>  void ioc_clear_queue(struct request_queue *q);
>  #else

The blk-ioc changes should go into there own patch, to separate block layer
changes and bfq scheduler changes. No ?


-- 
Damien Le Moal
Western Digital Research