From: Tejun Heo <tj@kernel.org>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Kent Overstreet <kmo@daterainc.com>, Jens Axboe <axboe@kernel.dk>,
Christoph Hellwig <hch@lst.de>,
"linux-kernel@vger.kernel.org >> Linux Kernel Mailing List"
<linux-kernel@vger.kernel.org>,
linux-s390 <linux-s390@vger.kernel.org>
Subject: Re: blk-mq vs cpu hotplug performance (due to percpu_ref_put performance)
Date: Tue, 28 Oct 2014 16:30:40 -0400 [thread overview]
Message-ID: <20141028203040.GD8205@htj.dyndns.org> (raw)
In-Reply-To: <544FFC9C.60908@de.ibm.com>
On Tue, Oct 28, 2014 at 09:29:16PM +0100, Christian Borntraeger wrote:
> 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?
Yeah, yeah, I was confused. We just need to initiate the killing for
all mqs at once and then wait for the completions. Shouldn't be too
difficult to fix. Will get to it soon.
Thanks.
--
tejun
next prev parent reply other threads:[~2014-10-28 20:30 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-28 19:35 blk-mq vs cpu hotplug performance (due to percpu_ref_put performance) Christian Borntraeger
2014-10-28 20:00 ` Tejun Heo
2014-10-28 20:20 ` Christian Borntraeger
2014-10-28 20:22 ` Tejun Heo
2014-10-28 20:26 ` Tejun Heo
2014-10-28 20:29 ` Christian Borntraeger
2014-10-28 20:30 ` Tejun Heo [this message]
2014-11-04 18:52 ` [PATCH block/for-linus] blk-mq: make mq_queue_reinit_notify() freeze queues in parallel Tejun Heo
2014-11-04 19:46 ` Christian Borntraeger
2014-11-04 21:48 ` Jens Axboe
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