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().
>
> Or is something else going on here?
>
> Thanx, Paul
>
> > return; /* We cleared it, done! */
> > }
> > oldstate = newoldstate;
> > --
> > 1.8.3.1
> >
>
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