On Wed, Dec 04, 2013 at 06:50:37PM -0800, Paul E. McKenney wrote: > On Thu, Dec 05, 2013 at 02:20:55AM +0100, Frederic Weisbecker wrote: > > On Wed, Dec 04, 2013 at 11:39:57AM -0800, Paul E. McKenney wrote: > > > Hello, Frederic, > > > > > > Just realized that I could further decrease RT latency of one of my "shut > > > up RCU on NO_HZ_FULL CPUs" patches if I relied on CPU 0 always having > > > a scheduling-clock tick unless the entire system is idle. The trick > > > is that I could then rely on CPU 0 to detect RCU CPU stall warnings, > > > and remove the checking from the other CPUs. > > > > > > Thoughts? > > > > You're right on time as I'm currently working on that :) > > So the plan is to allow timekeeping to be handled by a set of CPUs > > (cpu_housekeeping_mask > > which I guess should be ~nohz_full_mask & cpu_online_mask). I think it will > > be better > > for powersaving. I guess you could balance the RCU stall checks in this > > set of housekeeping CPUs? > > > > It should be very easy to make the rcu sysidle stuff to support that > > housekeeping set, > > I just looked into it and all we need to do is to turn the several "cpu == > > tick_do_timer" > > checks into something like is_housekeeping_cpu(cpu). And may be a few easy > > details, like which > > CPU from the housekeeping set should get the kick IPI, well the first one > > available should be a good start, > > of course I expect some issues with cpu hotplug. > > But other than that, RCU sysidle detection is mostly ready to support > > tracking only a given subset > > of CPUs instead of all of them. That's in fact what it already does > > currently by excluding the > > fixed boot timekeeping CPU. > > > > So I'm working on that and should have some patches ready soon. > > Thank you for the info! Nice to know that RCU will continue to be able > to rely on there being at least one housekeeping CPU. ;-) > > At that point, tick_nohz_full_cpu() would still be a good way for RCU > to distinguish housekeeping CPUs from working CPUs, correct?
Correct! > > > In fact I just realized that all the sysidle detection infrastructure is > > there and working > > but we forgot to plug it in the tick engine, and thus we are still running > > with periodic CPU 0 even with CONFIG_NO_HZ_FULL_SYSIDLE=y. Anyway I have a > > few changes > > ready to enable that, lets hope testing will be ok :) > > Indeed! ;-) > > The CONFIG_NO_HZ_FULL_SYSIDLE=y might complicate things a bit. But I > guess the problem would be a corner case -- the system entered sysidle > mode with a grace period pending, which should eventually wake up the > corresponding grace-period kthread, which might be prevented from ever > running due to high load or something. If that problem arises, I will > fix it. I see. Well we'll find out. In the meantime I successfully plugged sysidle detection with full dynticks and it surprisingly works like a charm. Which makes me think there must be some bug in my patches that make things working by accident :) I'll post soon. Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/