linux-kernel - Re: [PATCH 04/10] nohz: Appropriate timekeeper kick on sysidle break

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Date:	Mon, 21 Jul 2014 11:15:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 04/10] nohz: Appropriate timekeeper kick on sysidle break

On Sat, Jul 19, 2014 at 02:44:15AM +0200, Frederic Weisbecker wrote:
> When a CPU wakes up from idle and finds out that the timekeeper is
> sleeping, we need to kick it such that it switches from dynticks to
> periodic mode to maintain its timekeeping duty on behalf of the newly
> awoken CPU.
> 
> However we aren't using the right API for that. rcu_kick_nohz_cpu() is
> aimed at waking full dynticks CPUs and not the timekeeper.
> 
> Moreover the timekeeper must perform a new dynticks cycle to check the
> new sysidle state and restart the tick if necessary. A simple call
> to irq_exit() isn't enough.
> 
> wake_up_nohz_cpu() is a good fit for that job because it pulls the
> target out of the idle loop and restart the tick. Then if no other
> task waits for the CPU, it will reenter the idle loop and then the
> new sysidle state will be visible and well handled.
> 
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>

Either name is fine by me...  ;-)

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  kernel/rcu/tree_plugin.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 395c14d..b65da1a 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2494,7 +2494,7 @@ void rcu_sysidle_force_exit(void)
>  				      oldstate, RCU_SYSIDLE_NOT);
>  		if (oldstate == newoldstate &&
>  		    oldstate == RCU_SYSIDLE_FULL_NOTED) {
> -			rcu_kick_nohz_cpu(tick_do_timer_cpu);
> +			wake_up_nohz_cpu(tick_do_timer_cpu);
>  			return; /* We cleared it, done! */
>  		}
>  		oldstate = newoldstate;
> -- 
> 1.8.3.1
> 
> --
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