linux-kernel - Re: [PATCH 11/13] nohz: Wake up timekeeper on exit from sysidle state

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Date:	Tue, 17 Dec 2013 15:34:54 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Stultz <john.stultz@...aro.org>,
	Alex Shi <alex.shi@...aro.org>,
	Kevin Hilman <khilman@...aro.org>
Subject: Re: [PATCH 11/13] nohz: Wake up timekeeper on exit from sysidle state

On Tue, Dec 17, 2013 at 11:51:30PM +0100, Frederic Weisbecker wrote:
> When a full dynticks CPU wakes up from sysidle state, which means that
> all full dynticks CPUs were previously sleeping, it's possible that
> all the potential timekeeping CPUs are sleeping as well and nobody
> maintains the associated duty.
> 
> But full dynticks CPUs don't run the tick by definition so we need
> to wake up a timekeeper such that it can handle the timekeeping
> duty on behalf of the freshly awoken full dyntick CPU.
> 
> To achieve this and ensure that this CPU won't deal with stale
> jiffies values, lets wake up the default timekeeper using the right
> API.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: Alex Shi <alex.shi@...aro.org>
> Cc: Kevin Hilman <khilman@...aro.org>
> ---
>  kernel/rcu/tree_plugin.h | 2 +-
>  kernel/time/tick-sched.c | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 1795265..b43e32d 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_timekeeping_default_cpu());
> +			tick_nohz_full_kick_timekeeping();

OK, I guess I should look at the patches in order.  So yes, it is no
longer safe to just kick tick_do_timer_cpu.  ;-)

Never mind my question on patch 12/13 in this series.

							Thanx, Paul

>  			return; /* We cleared it, done! */
>  		}
>  		oldstate = newoldstate;
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 94b6901..f5ae69f 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -302,7 +302,8 @@ void tick_nohz_full_kick_all(void)
>  /**
>   * tick_nohz_full_kick_timekeeping - kick the default timekeeper
>   *
> - * kick the default timekeeper when a secondary timekeeper goes offline.
> + * kick the default timekeeper when full dynticks CPUs exit full
> + * system idle state or when a secondary timekeeper goes offline.
>   */
>  void tick_nohz_full_kick_timekeeping(void)
>  {
> -- 
> 1.8.3.1
> 

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