Re: [PATCH] a local-timer-free version of RCU

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Nov 16, 2010 at 07:51:04AM -0800, Paul E. McKenney wrote:
> On Tue, Nov 16, 2010 at 02:52:34PM +0100, Frederic Weisbecker wrote:
> > On Mon, Nov 15, 2010 at 05:28:46PM -0800, Paul E. McKenney wrote:
> > > My concern is not the tick -- it is really easy to work around lack of a
> > > tick from an RCU viewpoint.  In fact, this happens automatically given the
> > > current implementations!  If there is a callback anywhere in the system,
> > > then RCU will prevent the corresponding CPU from entering dyntick-idle
> > > mode, and that CPU's clock will drive the rest of RCU as needed via
> > > force_quiescent_state().
> > 
> > Now, I'm confused, I thought a CPU entering idle nohz had nothing to do
> > if it has no local callbacks, and rcu_enter_nohz already deals with
> > everything.
> > 
> > There is certainly tons of subtle things in RCU anyway :)
> 
> Well, I wasn't being all that clear above, apologies!!!
> 
> If a given CPU hasn't responded to the current RCU grace period,
> perhaps due to being in a longer-than-average irq handler, then it
> doesn't necessarily need its own scheduler tick enabled.  If there is a
> callback anywhere else in the system, then there is some other CPU with
> its scheduler tick enabled.  That other CPU can drive the slow-to-respond
> CPU through the grace-period process.



So, the scenario is that a first CPU (CPU 0) enqueues a callback and then
starts a new GP. But the GP is abnormally long because another CPU (CPU 1)
takes too much time to respond. But the CPU 2 enqueues a new callback.

What you're saying is that CPU 2 will take care of the current grace period
that hasn't finished, because it needs to start another one?
So this CPU 2 is going to be more insistant and will then send IPIs to
CPU 1.

Or am I completely confused? :-D

Ah, and if I understood well, if nobody like CPU 2 had been starting a new
grace period, then nobody would send those IPIs?

Looking at the rcu tree code, the IPI is sent from the state machine in
force_quiescent_state(), if the given CPU is not in dyntick mode.
And force_quiescent_state() is either called from the rcu softirq
or when one queues a callback. So, yeah, I think I understood correctly :)

But it also means that if we have two CPUs only, and CPU 0 starts a grace
period and then goes idle. CPU 1 may never respond and the grace period
may end in a rough while.



> The current RCU code should work in the common case.  There are probably
> a few bugs, but I will make you a deal.  You find them, I will fix them.
> Particularly if you are willing to test the  fixes.


Of course :)


 
> > > The force_quiescent_state() workings would
> > > want to be slightly different for dyntick-hpc, but not significantly so
> > > (especially once I get TREE_RCU moved to kthreads).
> > > 
> > > My concern is rather all the implicit RCU-sched read-side critical
> > > sections, particularly those that arch-specific code is creating.
> > > And it recently occurred to me that there are necessarily more implicit
> > > irq/preempt disables than there are exception entries.
> > 
> > Doh! You're right, I don't know why I thought that adaptive tick would
> > solve the implicit rcu sched/bh cases, my vision took a shortcut.
> 
> Yeah, and I was clearly suffering from a bit of sleep deprivation when
> we discussed this in Boston.  :-/



I suspect the real problem was my oral english understanding ;-)



> > > 3.	The implicit RCU-sched read-side critical sections just work
> > > 	as they do today.
> > > 
> > > Or am I missing some other problems with this approach?
> > 
> > No, looks good, now I'm going to implement/test a draft of these ideas.
> > 
> > Thanks a lot!
> 
> Very cool, and thank you!!!  I am sure that you will not be shy about
> letting me know of any RCU problems that you might encounter.  ;-)


Of course not ;-)

Thanks!!