Re: [PATCH 06/13] nohz: Introduce full dynticks' default timekeepingtarget

[Paul E. McKenney]

On Tue, Dec 17, 2013 at 11:51:25PM +0100, Frederic Weisbecker wrote:
> When a full dynticks CPU wakes up while the whole rest of the system
> is idle, we need to wake up the CPU in charge of the timekeeping duty
> handling.
> 
> As of today, the CPU that maintains this duty is CPU 0 when
> CONFIG_NO_HZ_FULL=y. So referring to tick_do_timer_cpu like we
> currently do is correct. But this behaviour is subject to change
> in the future because we want to balance the timekeeping duty over all
> the CPUs outside the full dynticks range.
> 
> As such we now need to define a default timekeeping CPU which receives
> the timekeeping wakeup IPIs and which can't be offlined so that it's
> guaranteed to always be present for full dynticks CPUs housekeeping.
> 
> So lets stick to CPU 0 for this purpose. It's convenient because
> rejecting any other CPU's offlining request may result in suspend
> failure.

OK, so not multiple CPUs yet.  Whew!

Well, at least you know some of my concerns with multiple timekeeping
CPUs beforehand, which would not have happened had I reviewed the
patches in order.  ;-)

							Thanx, Paul

> We can optimize this solution later by adaptively sending the IPI
> to a potential timekeeping CPU that is already running a non idle task.
> 
> 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>
> ---
>  include/linux/tick.h     | 16 ++++++++++++++++
>  kernel/rcu/tree_plugin.h |  4 ++--
>  kernel/time/tick-sched.c |  2 +-
>  3 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index cf2fd34..af98d2c 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -180,6 +180,22 @@ static inline bool tick_nohz_full_cpu(int cpu)
>  }
> 
>  /**
> + * tick_timekeeping_default_cpu - seek timekeeping default CPU
> +
> + * @return the default target which we send an IPI to
> + * when a full dynticks CPU wakes up and exits from full
> + * system idle state.
> + *
> + * This target is always CPU 0 in full dynticks environment.
> + * If we were to pick up any other CPU, that would result in suspend
> + * failures due to rejected offlining request.
> + */
> +static inline int tick_timekeeping_default_cpu(void)
> +{
> +	return 0;
> +}
> +
> +/**
>   * tick_timeeping_cpu - check if a CPU is elligble to handle timekeeping duty
>   * @cpu:	the cpu to check
>   *
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 84d90c8..1795265 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) {
> -			smp_send_reschedule(tick_do_timer_cpu);
> +			smp_send_reschedule(tick_timekeeping_default_cpu());
>  			return; /* We cleared it, done! */
>  		}
>  		oldstate = newoldstate;
> @@ -2597,7 +2597,7 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp)
>   */
>  static void rcu_bind_gp_kthread(void)
>  {
> -	int cpu = ACCESS_ONCE(tick_do_timer_cpu);
> +	int cpu = tick_timekeeping_default_cpu();
> 
>  	if (cpu < 0 || cpu >= nr_cpu_ids)
>  		return;
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index ea0d411..9a91c31 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -305,7 +305,7 @@ static int tick_nohz_cpu_down_callback(struct notifier_block *nfb,
>  		 * If we handle the timekeeping duty for full dynticks CPUs,
>  		 * we can't safely shutdown that CPU.
>  		 */
> -		if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
> +		if (tick_nohz_full_running && tick_timekeeping_default_cpu() == cpu)
>  			return NOTIFY_BAD;
>  		break;
>  	}
> -- 
> 1.8.3.1
> 

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