linux-kernel - Re: [ANNOUNCE] 3.7-nohz1

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Date:	Sat, 5 Jan 2013 00:42:53 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Li Zhong <zhong@...ux.vnet.ibm.com>
Subject: Re: [ANNOUNCE] 3.7-nohz1

2012/12/30 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> On Mon, Dec 24, 2012 at 12:43:25AM +0100, Frederic Weisbecker wrote:
>> 2012/12/21 Steven Rostedt <rostedt@...dmis.org>:
>> > 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.

Thanks.
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