linux-kernel - Re: [PATCH RFC nohz_full 6/7] nohz_full: Add full-system-idle state machine

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Message-ID: <20130718142450.GB7398@somewhere>
Date:	Thu, 18 Jul 2013 16:24:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, sbw@....edu
Subject: Re: [PATCH RFC nohz_full 6/7] nohz_full: Add full-system-idle state
 machine

On Wed, Jul 17, 2013 at 08:39:21PM -0700, Paul E. McKenney wrote:
> On Thu, Jul 18, 2013 at 03:33:01AM +0200, Frederic Weisbecker wrote:
> > So it's like:
> > 
> >     CPU 0                                              CPU 1
> > 
> >     read I                                             write I
> >     smp_mb()                                           smp_mb()
> >     cmpxchg S                                          read S
> > 
> > I still can't find what guarantees we don't read a value in CPU 1 that is way below
> > what we want.
> 
> One key point is that there is a second cycle from LONG to FULL.
> 
> (Not saying that there is not a bug -- there might well be.  In fact,
> I am starting to think that I need to do another Promela model...

Now I'm very confused :)

I'm far from being a specialist on these matters but I would really love to
understand this patchset. Is there any documentation somewhere I can read
that could help, something about cycles of committed memory or something?


> 
> > > Unfortunately, the reasoning in #2 above does not hold in the small-CPU
> > > case because there is the possibility of both the timekeeping CPU and
> > > the RCU grace-period kthread concurrently advancing the state machine.
> > > This would be bad, good catch!!!
> > 
> > It's not like I spotted anything myself but you're welcome :)
> 
> I will take them any way I can get them.  ;-)
> 
> > > The patch below (untested) is an attempt to fix this.  If it actually
> > > works, I will merge it in with 6/7.
> > > 
> > > Anything else I missed?  ;-)
> > 
> > Well I guess I'll wait one more night before trying to understand
> > the below ;)
> 
> The key point is that the added check means that either the timekeeping
> CPU is advancing the state machine (if there are few CPUs) or the
> RCU grace-period kthread is (if there are many CPUs), but never both.
> Or that is the intent, anyway!

Yeah got that.

Thanks!
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