Re: [PATCH 10/13] nohz: Hand over timekeeping duty on cpu offlining

[Paul E. McKenney]

On Tue, Dec 17, 2013 at 11:51:29PM +0100, Frederic Weisbecker wrote:
> When there are full dynticks CPUs around and the timekeeper goes
> offline, we have to hand over the timekeeping duty to another potential
> timekeeper.
> 
> The default timekeeper (aka CPU 0) is the perfect candidate for this
> task since it can't be offlined itself.
> 
> So lets send an IPI to the default timekeeping when the current
> timekeeper goes offline, so that the duty is relayed.

A few comments below.

							Thanx, Paul

> 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     |  2 ++
>  kernel/time/tick-sched.c | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index af98d2c..bd3c32e 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -218,6 +218,7 @@ extern void tick_nohz_init(void);
>  extern void __tick_nohz_full_check(void);
>  extern void tick_nohz_full_kick(void);
>  extern void tick_nohz_full_kick_all(void);
> +extern void tick_nohz_full_kick_timekeeping(void);
>  extern void __tick_nohz_task_switch(struct task_struct *tsk);
>  # else
>  static inline void tick_nohz_init(void) { }
> @@ -227,6 +228,7 @@ static inline bool tick_timekeeping_cpu(int cpu) { return true; }
>  static inline void __tick_nohz_full_check(void) { }
>  static inline void tick_nohz_full_kick(void) { }
>  static inline void tick_nohz_full_kick_all(void) { }
> +static inline void tick_nohz_full_kick_timekeeping(void) { }
>  static inline void __tick_nohz_task_switch(struct task_struct *tsk) { }
>  #endif
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 527b501..94b6901 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -217,6 +217,12 @@ static u64 tick_timekeeping_max_deferment(struct tick_sched *ts)
>  		return timekeeping_max_deferment();
> 
>  	/*
> +	 * Order tick_do_timer_cpu read against the IPI, pairs with
> +	 * tick_nohz_full_kick_timekeeping()
> +	 */
> +	smp_rmb();

If this is the handler for the smp_send_reschedule(), then the above
memory barrier is not needed.  (See my comment below.)

> +
> +	/*
>  	 * If we are the timekeeper and all full dynticks CPUs are idle,
>  	 * then we can finally sleep.
>  	 */
> @@ -293,6 +299,22 @@ void tick_nohz_full_kick_all(void)
>  	preempt_enable();
>  }
> 
> +/**
> + * tick_nohz_full_kick_timekeeping - kick the default timekeeper
> + *
> + * kick the default timekeeper when a secondary timekeeper goes offline.
> + */
> +void tick_nohz_full_kick_timekeeping(void)
> +{
> +	tick_do_timer_cpu = tick_timekeeping_default_cpu();
> +	/*
> +	 * Order tick_do_timer_cpu against the IPI, pairs with
> +	 * tick_timekeeping_max_deferment on irq exit.
> +	 */
> +	smp_wmb();

But the IPI is supposed to provide full ordering between the CPU invoking
the IPI and the IPI handler, right?  I do not believe that you need
the above smp_wmb() -- though keeping the comment stating that you are
relying on the implicit barrier in IPI would be good.

> +	smp_send_reschedule(tick_timekeeping_default_cpu());

Again, smp_send_reschedule()'s IPI hander does not necessarily do
anything if there is nothing for the scheduler to do, so any needed
actions are taking in the return-from-interrupt code?

> +}
> +
>  /*
>   * Re-evaluate the need for the tick as we switch the current task.
>   * It might need the tick due to per task/process properties:
> @@ -351,6 +373,15 @@ static int tick_nohz_cpu_down_callback(struct notifier_block *nfb,
>  		if (tick_nohz_full_running && tick_timekeeping_default_cpu() == cpu)
>  			return NOTIFY_BAD;
>  		break;
> +
> +	case CPU_DYING:
> +		/*
> +		 * Notify default timekeeper if we are giving up
> +		 * timekeeping duty
> +		 */
> +		if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
> +			tick_nohz_full_kick_timekeeping();
> +		break;
>  	}
>  	return NOTIFY_OK;
>  }
> -- 
> 1.8.3.1
> 

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