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

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Message-ID: <alpine.LFD.2.02.1206141656040.3086@ionos>
Date:	Thu, 14 Jun 2012 17:02:11 +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 03:32:19PM +0200, 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.
> 
> A simple offline was triggering it for me.  Perhaps some of my debug
> code was inappropriate, will retry.
> 
> > > > 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 ?
> 
> We rely on their own sense of self-preservation preventing them from
> getting involved in such insanity.
> 
> > > > 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?
> 
> It also boosts the priority of preempted RCU read-side critical sections.

Though the only way how this thread is invoked is via the timeout of
that yield timer. So I really have a hard time for understanding that.

cpu_kthread()
    ....
    yield()
		timer fires -> mark cpu in mask and wakeup node kthread

  node_kthread()		
      do magic boost invocation
      push priority of cpu_kthread marked in mask

For the boost thread it looks like:

boost_kthread()
    .....
    yield()
		timer fires -> wakeup node kthread

  node_kthread()		
      do magic boost invocation, but no prio adjustment of boost thread.

/me scratches head
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