Re: [PATCH 05/13] rcu: Fix unraised IPI to timekeeping CPU

[Frederic Weisbecker]

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 <fweisbec@xxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Cc: John Stultz <john.stultz@xxxxxxxxxx>
> > Cc: Alex Shi <alex.shi@xxxxxxxxxx>
> > Cc: Kevin Hilman <khilman@xxxxxxxxxx>
> > ---
> >  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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