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);
}
/*
@@ -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 */
+ 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
--
2.39.2
