Am 04.11.2014 19:52, schrieb Tejun Heo:
> 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>
Tested-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Thanks.
> ---
> 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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