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

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Message-ID: <20141028202637.GC8205@htj.dyndns.org>
Date:	Tue, 28 Oct 2014 16:26:37 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
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)

On Tue, Oct 28, 2014 at 04:22:55PM -0400, Tejun Heo wrote:
> 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_*().

I got confused, so perpcu_ref does wait for the async grace period
making it synchronous.  I see what you mean.  This isn't during init
but freezing itself being slow.  Hmmmm... so are you seeing multiple
queues doing that back-to-back?  If so, the right thing to do would be
making the freezing take place in parallel.  I'll look into it.

Thanks.

-- 
tejun
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