Re: blk-mq vs cpu hotplug performance (due to percpu_ref_put performance)

[Tejun Heo]

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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