On Wed, Dec 18, 2013 at 03:13:52PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 17, 2013 at 03:21:00PM -0800, Paul E. McKenney wrote:
> > On Tue, Dec 17, 2013 at 11:51:24PM +0100, Frederic Weisbecker wrote:
> > > The plan with full system idle detection is to allow the timekeeper
> > > to sleep when all full dynticks CPUs are sleeping.
> > >
> > > Then when a full dynticks CPU wakes up while the whole system is idle,
> > > it sends an IPI to the timekeeping CPU which then restarts its tick
> > > and polls on its timekeeping duty on behalf of all other CPUs in the
> > > system.
> > >
> > > But we are using rcu_kick_nohz_cpu() to raise this IPI, which is wrong
> > > because this function is used to kick full dynticks CPUs when they run
> > > in the kernel for too long without reporting a quiescent state. And
> > > this function ignores targets that are not full dynticks, like our
> > > timekeeper.
> > >
> > > To fix this, use the smp_send_reschedule() function directly.
> >
> > I guess the fact that you needed some change is reassuring. You know
> > the old saying, "no bugs, no users". ;-)
>
> ;-)
>
> >
> > > Signed-off-by: Frederic Weisbecker <fweis...@gmail.com>
> > > Cc: Thomas Gleixner <t...@linutronix.de>
> > > Cc: Ingo Molnar <mi...@kernel.org>
> > > Cc: Peter Zijlstra <pet...@infradead.org>
> > > Cc: Steven Rostedt <rost...@goodmis.org>
> > > Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > > Cc: John Stultz <john.stu...@linaro.org>
> > > Cc: Alex Shi <alex....@linaro.org>
> > > Cc: Kevin Hilman <khil...@linaro.org>
> > > ---
> > > kernel/rcu/tree_plugin.h | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 08004da..84d90c8 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -2488,7 +2488,7 @@ void rcu_sysidle_force_exit(void)
> > > oldstate, RCU_SYSIDLE_NOT);
> > > if (oldstate == newoldstate &&
> > > oldstate == RCU_SYSIDLE_FULL_NOTED) {
> > > - rcu_kick_nohz_cpu(tick_do_timer_cpu);
> > > + smp_send_reschedule(tick_do_timer_cpu);
> >
> > Hmmm...
> >
> > We haven't done any sort of wakeup, and tick_nohz_full_cpu() should
> > return false for tick_do_timer_cpu, and I don't see that we have
> > done anything to make got_nohz_idle_kick() return true.
> >
> > So the idea is that the fact of the interrupt is sufficient, and
> > the target CPU will figure out that it must turn its scheduling-clock
> > interrupt when returning from interrupt?
>
> Exactly, the interrupt alone is sufficient and the tick is reevaluated
> on irq_exit().
But if that is the case, why do you need the change to scheduler_ipi()
in patch 07/13? Just having received any sort of IPI should suffice.
Thanx, Paul
> > Or is something else going on here?
> >
> > Thanx, Paul
> >
> > > return; /* We cleared it, done! */
> > > }
> > > oldstate = newoldstate;
> > > --
> > > 1.8.3.1
> > >
> >
>
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