linux-kernel - Re: [PATCH block/for-linus] blk-mq: make mq_queue_reinit

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Date:	Tue, 04 Nov 2014 14:48:47 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>
CC:	Kent Overstreet <kmo@...erainc.com>,
	Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List" 
	<linux-kernel@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH block/for-linus] blk-mq: make mq_queue_reinit_notify()
 freeze queues in parallel

On 2014-11-04 11:52, Tejun Heo wrote:
> 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@...nel.org>
> Reported-by: Christian Borntraeger <borntraeger@...ibm.com>
> ---
> 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?

Will queue up for 3.18.

-- 
Jens Axboe

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