Re: [PATCH 12/13] nohz: Allow all CPUs outside nohz_full range to do timekeeping

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

On Tue, Dec 17, 2013 at 11:51:31PM +0100, Frederic Weisbecker wrote:
> Now that we have all the infrastructure in place and ready to support
> timekeeping duty balanced across every non full dynticks CPUs, we can
> hereby extend the timekeeping duty affinity.
> 
> 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>
> ---
>  include/linux/tick.h | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index bd3c32e..07c02e8 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -203,15 +203,7 @@ static inline int tick_timekeeping_default_cpu(void)
>   */
>  static inline bool tick_timekeeping_cpu(int cpu)
>  {
> -	/*
> -	 * If there are full dynticks CPUs around,
> -	 * CPU 0 must stay periodic to update timekeeping.
> -	 */
> -	if (tick_nohz_full_enabled())
> -		return cpu == 0;
> -
> -	/* Otherwise any CPU is elligible for timekeeping duty */
> -	return true;
> +	return !tick_nohz_full_cpu(cpu);

OK, I guess the future is already here.  ;-)

Is it still OK for RCU to kick tick_do_timer_cpu?  Or are there race
conditions that could result in the wrong CPU being kicked?  Or is
there some guarantee that I missed that says that the timekeeping
CPU cannot change while in RCU_SYSIDLE_FULL_NOTED state?

							Thanx, Paul

>  }
> 
>  extern void tick_nohz_init(void);
> -- 
> 1.8.3.1
>