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 <fweis...@gmail.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Ingo Molnar <mi...@kernel.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Cc: John Stultz <john.stu...@linaro.org>
> Cc: Alex Shi <alex....@linaro.org>
> Cc: Kevin Hilman <khil...@linaro.org>
> ---
> 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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