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.
>
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