Messages in this thread | | | Date | Tue, 10 Dec 2013 07:08:46 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Will CPU 0 be forever prohibited from NO_HZ_FULL status? |
| |
On Tue, Dec 10, 2013 at 03:37:03PM +0100, Frederic Weisbecker wrote: > 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!
Cool! Maybe I should start future-proofing RCU in that manner.
> > > 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 :)
Must be some mistake! ;-)
Thanx, Paul
> I'll post soon. > > Thanks. >
| |