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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Alex Shi <alex.shi@linaro.org>
> Cc: Kevin Hilman <khilman@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?

Or is something else going on here?

							Thanx, Paul

>  			return; /* We cleared it, done! */
>  		}
>  		oldstate = newoldstate;
> -- 
> 1.8.3.1
>