linux-kernel - Re: [PATCH 07/13] sched: Enable IPI reception on timekeeper under nohz full system

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Date:	Tue, 17 Dec 2013 15:52:48 -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 07/13] sched: Enable IPI reception on timekeeper under
 nohz full system

On Tue, Dec 17, 2013 at 11:51:26PM +0100, Frederic Weisbecker wrote:
> We need the default timekeeping CPU to be able to receive IPIs sent
> from full dynticks CPUs when they wake up from full system idle state.
> 
> Therefore we need an entrypoint from the scheduler IPI so that the
> need to poll on timekeeping duty is re-evaluated from irq_exit().
> 
> In order to achieve this, lets take the scheduler IPI everytime as long
> as there is at least one full dynticks CPU around. Full dynticks CPUs
> are interested too in taking scheduler IPIs to reevaluate their tick.
> 
> 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/sched/core.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e85cda2..f46a7bc 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1502,9 +1502,9 @@ void scheduler_ipi(void)
>  	if (tif_need_resched())
>  		set_preempt_need_resched();
> 
> -	if (llist_empty(&this_rq()->wake_list)
> -			&& !tick_nohz_full_cpu(smp_processor_id())
> -			&& !got_nohz_idle_kick())
> +	if (llist_empty(&this_rq()->wake_list) &&
> +	    !tick_nohz_full_enabled() &&
> +	    !got_nohz_idle_kick())
>  		return;

OK, this is what I was missing in my question about whether the
NO_HZ_FULL state was re-evaluated in the interrupt-return path.

						Thanx, Paul

>  	/*
> -- 
> 1.8.3.1
> 

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