Re: Will CPU 0 be forever prohibited from NO_HZ_FULL status?

[Frederic Weisbecker <fweisbec@gmail.com>]

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.