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 <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 | 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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