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

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Date:	Thu, 14 Jun 2012 15:32:19 +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 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?

Thanks,

	tglx

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