linux-kernel - Re: [PATCH RT 4/6] rt/locking: Reenable migration accross schedule

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Date:	Fri, 25 Mar 2016 09:52:49 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RT 4/6] rt/locking: Reenable migration accross schedule

On Fri, 25 Mar 2016, Mike Galbraith wrote:
> On Thu, 2016-03-24 at 12:06 +0100, Mike Galbraith wrote:
> > On Thu, 2016-03-24 at 11:44 +0100, Thomas Gleixner wrote:
> > >  
> > > > On the bright side, with the busted migrate enable business reverted,
> > > > plus one dinky change from me [1], master-rt.today has completed 100
> > > > iterations of Steven's hotplug stress script along side endless
> > > > futexstress, and is happily doing another 900 as I write this, so the
> > > > next -rt should finally be hotplug deadlock free.
> > > > 
> > > > Thomas's state machinery seems to work wonders.  'course this being
> > > > hotplug, the other shoe will likely apply itself to my backside soon.
> > > 
> > > That's a given :)
> > 
> > blk-mq applied it shortly after I was satisfied enough to poke xmit.
> 
> The other shoe is that notifiers can depend upon RCU grace periods, so
> when pin_current_cpu() snags rcu_sched, the hotplug game is over.
> 
> blk_mq_queue_reinit_notify:
>         /*
>          * We need to freeze and reinit all existing queues.  Freezing
>          * involves synchronous wait for an RCU grace period and doing it
>          * one by one may take a long time.  Start freezing all queues in
>          * one swoop and then wait for the completions so that freezing can
>          * take place in parallel.
>          */
>         list_for_each_entry(q, &all_q_list, all_q_node)
>                 blk_mq_freeze_queue_start(q);
>         list_for_each_entry(q, &all_q_list, all_q_node) {
>                 blk_mq_freeze_queue_wait(q);

Yeah, I stumbled over that already when analysing all the hotplug notifier
sites. That's definitely a horrible one.
 
> Hohum (sharpens rock), next.

/me recommends frozen sharks

Thanks,

       tglx