Re: [ANNOUNCE] 3.7-nohz1

From: Frederic Weisbecker
Date: Fri Jan 04 2013 - 18:42:43 EST

2012/12/30 Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>:
> On Mon, Dec 24, 2012 at 12:43:25AM +0100, Frederic Weisbecker wrote:
>> 2012/12/21 Steven Rostedt <rostedt@xxxxxxxxxxx>:
>> > On Thu, 2012-12-20 at 19:32 +0100, Frederic Weisbecker wrote:
>> >> Let's imagine you have 4 CPUs. We keep the CPU 0 to offline RCU callbacks there and to
>> >> handle the timekeeping. We set the rest as full dynticks. So you need the following kernel
>> >> parameters:
>> >>
>> >> rcu_nocbs=1-3 full_nohz=1-3
>> >>
>> >> (Note rcu_nocbs value must always be the same as full_nohz).
>> >
>> > Why? You can't have: rcu_nocbs=1-4 full_nohz=1-3
>> That should be allowed.
>> > or: rcu_nocbs=1-3 full_nohz=1-4 ?
>> But that not.
>> You need to have: rcu_nocbs & full_nohz == full_nohz. This is because
>> the tick is not there to maintain the local RCU callbacks anymore. So
>> this must be offloaded to the rcu_nocb threads.
>> I just have a doubt with rcu_nocb. Do we still need the tick to
>> complete the grace period for local rcu callbacks? I need to discuss
>> that with Paul.
> The tick is only needed if rcu_needs_cpu() returns false. Of course,
> this means that if you don't invoke rcu_needs_cpu() before returning to
> adaptive-idle usermode execution, you are correct that a full_nohz CPU
> would also have to be a rcu_nocbs CPU.
> That said, I am getting close to having an rcu_needs_cpu() that only
> returns false if there are callbacks immediately ready to invoke, at
> least if RCU_FAST_NO_HZ=y.

Ok. Also when a CPU enqueues a callback and starts a grace period, the
tick polls on the grace period completion. How is it handled with
rcu_nocbs CPUs? Does rcu_needs_cpu() return false until the grace
period is completed? If so I still need to restart the local tick
whenever a new callback is enqueued.

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