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

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Date:	Thu, 14 Jun 2012 08:01:18 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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 Thu, Jun 14, 2012 at 04:12:14PM +0200, Thomas Gleixner wrote:
> On Thu, 14 Jun 2012, Thomas Gleixner wrote:
> > On Thu, 14 Jun 2012, Thomas Gleixner wrote:
> > 
> > > On Thu, 14 Jun 2012, Paul E. McKenney wrote:
> > > > On Thu, Jun 14, 2012 at 01:20:39PM +0200, Thomas Gleixner wrote:
> > > > > I gave it a quick shot, but I was not able to reproduce the hang yet.
> > > > 
> > > > Really?  I have a strictly Western-Hemisphere bug?  ;-)
> > > 
> > > I guess I need to fire up rcu torture to make it surface.
> > >  
> > > > > But looking at the thread function made me look into rcu_yield() and I
> > > > > really wonder what kind of drug induced that particular piece of
> > > > > horror.
> > > > 
> > > > When you are working on something like RCU priority boosting, no other
> > > > drug is in any way necessary.  ;-)
> > > 
> > > And how do we protect minors from that ?
> > >  
> > > > > I can't figure out why this yield business is necessary at all. The
> > > > > commit logs are as helpful as the missing code comments :)
> > > > > 
> > > > > I suspect that it's some starvation issue. But if we need it, then
> > > > > can't we replace it with something sane like the (untested) patch
> > > > > below?
> > > > 
> > > > Yep, starvation.  I will take a look at your approach after I wake
> > > > up a bit more.
> > > 
> > > Btw, if that simpler yield approach is working and I can't see why it
> > > shouldn't then you can get rid of the node task as well. The only
> > > purpose of it is to push up the priority of yielding tasks, right?
> > 
> > Ah, missed that it calls rcu_initiate_boost() as well....
> 
> And looking further, I really don't understand why it's doing
> that. That node thread is only woken by these weird yield timers.

If your patch works out, it indeed might be possible to get rid of
->node_kthread_task.  The ->boost_kthread_task needs to stay, however.

							Thanx, Paul

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