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Date: Sat, 29 Dec 2012 19:56:38 -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 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. Thanx, Paul > > That needs to be fixed. Either with a warning, and/or to force the two > > to be the same. That is, if they specify: > > > > rcu_nocbs=1-3 full_nohz=1-4 > > > > Then set rcu_nocbs=1-4 with a warning about it. Or simply set > > full_nohz=1-3. > > Yep, will do. > > Thanks! > > > > > -- Steve > > > >> > >> Now if you want proper isolation you need to: > >> > >> * Migrate your processes adequately > >> * Migrate your irqs to CPU 0 > >> * Migrate the RCU nocb threads to CPU 0. Example with the above configuration: > >> > >> for p in $(ps -o pid= -C rcuo1,rcuo2,rcuo3) > >> do > >> taskset -cp 0 $p > >> done > >> > >> Then run what you want on the full dynticks CPUs. For best results, run 1 task > >> per CPU, mostly in userspace and mostly CPU bound (otherwise more IO = more kernel > >> mode execution = more chances to get IPIs, tick restarted, workqueues, kthreads, etc...) > >> > >> This page contains a good reminder for those interested in CPU isolation: https://github.com/gby/linux/wiki > >> > >> But keep in mind that my tree is not yet ready for serious production. > >> > > > > > -- 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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