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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Jul 21, 2014 at 10:33:06AM -0700, Paul E. McKenney wrote:
> On Mon, Jul 21, 2014 at 07:04:59PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 21, 2014 at 08:57:41AM -0700, Paul E. McKenney wrote:
> > > On Sun, Jul 20, 2014 at 10:34:17PM +0200, Peter Zijlstra wrote:
> > > > On Sun, Jul 20, 2014 at 04:47:59AM -0700, Paul E. McKenney wrote:
> > > > > So we really have to have -all- the CPUs be idle to turn off the timekeeper.
> > > > 
> > > > That seems to be pretty unavoidable any which way around.
> > > 
> > > Hmmm...  The exception would be the likely common case where none of
> > > the CPUs are flagged as nohz_full= CPUs.  If we handled that case as
> > > if CONFIG_NO_HZ_FULL=n, we would have handled almost all of
> > > the problem.
> > 
> > You mean that is not currently the case? Yes that seems like a fairly
> > sane thing to do.
> 
> Hard to say -- need to see where Frederic is putting the call to
> rcu_sys_is_idle().  On the RCU side, I could potentially lower overhead
> by checking tick_nohz_full_enabled() in a few functions.

Yeah you definetly can.

Just put this in the very beginning of rcu_sys_is_idle():

            if (tick_nohz_full_enabled())
                return true;

That imply perhaps a more appropriate name like rcu_sys_need_timekeeper(),
with inverted condition.