On 7/22/25 4:24 PM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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
