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 <[email protected]>
>
> 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...
Other thoughts on this? We really should not be setting
CONFIG_NO_HZ_FULL_SYSIDLE by default until this is solved.
Thanx, Paul
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index ddad959a9132..eaa32e4c228d 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2789,8 +2789,13 @@ static void rcu_sysidle_exit(struct rcu_dynticks
> > *rdtp, int irq)
> > * system-idle state. This means that the timekeeping CPU must
> > * invoke rcu_sysidle_force_exit() directly if it does anything
> > * more than take a scheduling-clock interrupt.
> > + *
> > + * In addition if we are not a nohz_full= CPU, then when we are
> > + * non-idle we have our own tick, so we don't need the timekeeping
> > + * CPU to keep a tick on our behalf. We assume that the timekeeping
> > + * CPU is also a nohz_full= CPU.
> > */
> > - if (smp_processor_id() == tick_do_timer_cpu)
> > + if (!tick_nohz_full_cpu(smp_processor_id()))
> > return;
> >
> > /* Update system-idle state: We are clearly no longer fully idle! */
> > @@ -2810,11 +2815,11 @@ static void rcu_sysidle_check_cpu(struct rcu_data
> > *rdp, bool *isidle,
> >
> > /*
> > * If some other CPU has already reported non-idle, if this is
> > - * not the flavor of RCU that tracks sysidle state, or if this
> > - * is an offline or the timekeeping CPU, nothing to do.
> > + * not the flavor of RCU that tracks sysidle state, or if this is
> > + * an offline or !nohz_full= or the timekeeping CPU, nothing to do.
> > */
> > if (!*isidle || rdp->rsp != rcu_sysidle_state ||
> > - cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
> > + cpu_is_offline(rdp->cpu) || !tick_nohz_full_cpu(rdp->cpu))
> > return;
> > if (rcu_gp_in_progress(rdp->rsp))
> > WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
> >
>
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