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Date: Mon, 7 Jan 2013 05:06:32 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Frederic Weisbecker <fweisbec@...il.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 On Sat, Jan 05, 2013 at 12:42:53AM +0100, Frederic Weisbecker wrote: > 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. If RCU_FAST_NO_HZ=n, then yes, this is the case, but only for !rcu_nocbs CPUs. > 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. Each rcu_nocbs CPU has a kthread, and that kthread is responsible for making sure that any needed grace periods move forward. In mainline, this is done via CPU 0, which is required to be a !rcu_nocbs CPU. In -rcu, the no-CBs kthreads communicate with the grace-period kthread via the rcu_node tree, so that if all CPUs are rcu_nocbs CPUs, rcu_needs_cpu() will always return false, even if RCU_FAST_NO_HZ=n. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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