Re: [PATCH tip/core/rcu] Do not keep timekeeping CPU tick running for non-nohz_full= CPUs

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Jul 21, 2014 at 12:12:48AM +0200, Frederic Weisbecker wrote:
> On Sun, Jul 20, 2014 at 04:47:59AM -0700, Paul E. McKenney wrote:
> > On Sat, Jul 19, 2014 at 08:01:24PM +0200, Frederic Weisbecker wrote:
> > > On Sat, Jul 19, 2014 at 09:53:50AM -0700, Paul E. McKenney wrote:
> > > > If a non-nohz_full= CPU is non-idle, it will have a scheduling-clock
> > > > interrupt, and therefore doesn't need the timekeeping CPU to keep
> > > > its scheduling-clock interrupt going.  This commit therefore ignores
> > > > the idle state of non-nohz_full CPUs when determining whether or not
> > > > the timekeeping CPU can safely turn off its scheduling-clock interrupt.
> > > > 
> > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > 
> > > Unfortunately that's not how things work. Running a CPU tick doesn't necessarily
> > > imply to run the timekeeping duty.
> > > 
> > > Only the timekeeper can update the timekeeping. There is an exception though:
> > > the timekeeping is also updated by dynticks idle CPUs when they wake up in an
> > > interrupt from idle.
> > > 
> > > Here is in practice why it doesn't work:
> > > 
> > > So lets say CPU 0 is timekeeper, CPU 1 a non-nohz-full CPU and all others are full-nohz.
> > > CPU 0 is sleeping. CPU 1 wakes up from idle, so it has an uptodate timekeeping but then
> > > if it continues to execute further without waking up CPU 0, it risks stale timestamps.
> > > 
> > > This can be changed by allowing timekeeping duty from all non-nohz_full CPUs, that's
> > > the initial direction I took, but it involved a lot of complications and scalability
> > > issues.
> > 
> > So we really have to have -all- the CPUs be idle to turn off the timekeeper.
> > This won't make the battery-powered embedded guys happy...
> 
> I can imagine all sorts of solutions to solve this. None of them look simple
> though. And I'm really convinced this isn't worth until some user comes up
> and report me that 1) he seriously uses full dynticks and 2) he needs non-full-nohz
> CPUs other than CPU 0.
> 
> If 1 and 2 ever happen, I'll gladly work on this.

Does the thought of special-casing the situation where CONFIG_NO_HZ_FULL=y,
CONFIG_NO_HZ_FULL_SYSIDLE=y, and there are no nohz_full= CPUs make sense?

> > Other thoughts on this?  We really should not be setting
> > CONFIG_NO_HZ_FULL_SYSIDLE by default until this is solved.
> 
> Well it's better to save energy when all CPUs are idle than never.

Fair point!

								Thanx, Paul