linux-kernel - Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu hotplug threads

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Date:	Wed, 13 Jun 2012 20:56:18 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu
 hotplug threads

On Wed, 13 Jun 2012, Paul E. McKenney wrote:
> On Wed, Jun 13, 2012 at 11:00:54AM -0000, Thomas Gleixner wrote:
>
> So I am currently trying to apply this to RCU's per-CPU kthread.
> I don't believe that I need to mess with RCU's per-rcu_node kthread
> because it can just have its affinity adjusted when the first CPU
> onlines and the last CPU offlines for the corresponding rcu_node.
> 
> One question below about the order of parking.
> 
> Also, I have not yet figured out how this avoids a parked thread waking
> up while the CPU is offline, but I am probably still missing something.

If it's just a spurious wakeup then it goes back to sleep right away
as nothing cleared the park bit.

If something calls unpark(), then it's toast. I should put a warning
into the code somewhere to catch that case.

> > +void smpboot_park_threads(unsigned int cpu)
> > +{
> > +	struct smp_hotplug_thread *cur;
> > +
> > +	mutex_lock(&smpboot_threads_lock);
> > +	list_for_each_entry(cur, &hotplug_threads, list)
> 
> Shouldn't this be list_for_each_entry_reverse()?  Yes, the notifiers
> still run in the same order for both online and offline, but all uses
> of smpboot_park_threads() would be new, so should be OK with the
> proper ordering, right?

Duh, yes

Thanks,

	tglx
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