linux-kernel - Re: blk-mq vs cpu hotplug performance (due to percpu_ref

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Message-ID: <544FFC9C.60908@de.ibm.com>
Date:	Tue, 28 Oct 2014 21:29:16 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Kent Overstreet <kmo@...erainc.com>, Jens Axboe <axboe@...nel.dk>,
	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: blk-mq vs cpu hotplug performance (due to percpu_ref_put performance)

Am 28.10.2014 21:22, schrieb Tejun Heo:
> On Tue, Oct 28, 2014 at 09:20:55PM +0100, Christian Borntraeger wrote:
>> I have not verified this, but I guess what happens is:
>> hotplug
>> -> notify
>> -> blk_mq_queue_reinit_notify
>> -> blk_mq_queue_reinit
>> -> blk_mq_freeze_queue
>> -> percpu_ref_kill
>> -> percpu_ref_kill_and_confirm
>> -> __percpu_ref_switch_to_atomic
>> -> call_rcu_sched  
> 
> But call_rcu_sched() wouldn't show up as latency.  It's an async call
> unlike synchronize_*().

Right, but

blk_mq_freeze_queue

also contains

wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->mq_usage_counter));

Isnt that wait_event woken up at the end of the call_rcu_sched?

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