q->mq_usage_counter is a percpu_ref which is killed and drained when
the queue is frozen. On a CPU hotplug event, blk_mq_queue_reinit()
which involves freezing the queue is invoked on all existing queues.
Because percpu_ref killing and draining involve a RCU grace period,
doing the above on one queue after another may take a long time if
there are many queues on the system.
This patch splits out initiation of freezing and waiting for its
completion, and updates blk_mq_queue_reinit_notify() so that the
queues are frozen in parallel instead of one after another. Note that
freezing and unfreezing are moved from blk_mq_queue_reinit() to
blk_mq_queue_reinit_notify().
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
Christian, can you please verify that this resolves the latency issue
that you're seeing? Jens, can you please route this patch once
Christian confirms it?
Thanks!
block/blk-mq.c | 41 +++++++++++++++++++++++++++++++++--------
1 file changed, 33 insertions(+), 8 deletions(-)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -107,11 +107,7 @@ static void blk_mq_usage_counter_release
wake_up_all(&q->mq_freeze_wq);
}
-/*
- * Guarantee no request is in use, so we can change any data structure of
- * the queue afterward.
- */
-void blk_mq_freeze_queue(struct request_queue *q)
+static void blk_mq_freeze_queue_start(struct request_queue *q)
{
bool freeze;
@@ -123,9 +119,23 @@ void blk_mq_freeze_queue(struct request_
percpu_ref_kill(&q->mq_usage_counter);
blk_mq_run_queues(q, false);
}
+}
+
+static void blk_mq_freeze_queue_wait(struct request_queue *q)
+{
wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->mq_usage_counter));
}
+/*
+ * Guarantee no request is in use, so we can change any data structure of
+ * the queue afterward.
+ */
+void blk_mq_freeze_queue(struct request_queue *q)
+{
+ blk_mq_freeze_queue_start(q);
+ blk_mq_freeze_queue_wait(q);
+}
+
static void blk_mq_unfreeze_queue(struct request_queue *q)
{
bool wake;
@@ -1921,7 +1931,7 @@ void blk_mq_free_queue(struct request_qu
/* Basically redo blk_mq_init_queue with queue frozen */
static void blk_mq_queue_reinit(struct request_queue *q)
{
- blk_mq_freeze_queue(q);
+ WARN_ON_ONCE(!q->mq_freeze_depth);
blk_mq_sysfs_unregister(q);
@@ -1936,8 +1946,6 @@ static void blk_mq_queue_reinit(struct r
blk_mq_map_swqueue(q);
blk_mq_sysfs_register(q);
-
- blk_mq_unfreeze_queue(q);
}
static int blk_mq_queue_reinit_notify(struct notifier_block *nb,
@@ -1956,8 +1964,25 @@ static int blk_mq_queue_reinit_notify(st
return NOTIFY_OK;
mutex_lock(&all_q_mutex);
+
+ /*
+ * We need to freeze and reinit all existing queues. Freezing
+ * involves synchronous wait for an RCU grace period and doing it
+ * one by one may take a long time. Start freezing all queues in
+ * one swoop and then wait for the completions so that freezing can
+ * take place in parallel.
+ */
+ list_for_each_entry(q, &all_q_list, all_q_node)
+ blk_mq_freeze_queue_start(q);
+ list_for_each_entry(q, &all_q_list, all_q_node)
+ blk_mq_freeze_queue_wait(q);
+
list_for_each_entry(q, &all_q_list, all_q_node)
blk_mq_queue_reinit(q);
+
+ list_for_each_entry(q, &all_q_list, all_q_node)
+ blk_mq_unfreeze_queue(q);
+
mutex_unlock(&all_q_mutex);
return NOTIFY_OK;
}
--
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