linux-kernel - Re: [PATCH V2] cpuidle: Add missing checks to the exit condition of cpu_idle

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Date:	Thu, 22 Jan 2015 22:00:57 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
cc:	peterz@...radead.org, linuxppc-dev@...ts.ozlabs.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] cpuidle: Add missing checks to the exit condition of
 cpu_idle_poll()

On Wed, 21 Jan 2015, Preeti U Murthy wrote:

> cpu_idle_poll() is entered into when either the cpu_idle_force_poll is set or
> tick_check_broadcast_expired() returns true. The exit condition from
> cpu_idle_poll() is tif_need_resched().
> 
> However this does not take into account scenarios where cpu_idle_force_poll
> changes or tick_check_broadcast_expired() returns false, without setting
> the resched flag. So a cpu will be caught in cpu_idle_poll() needlessly,
> thereby wasting power. Add an explicit check on cpu_idle_force_poll and
> tick_check_broadcast_expired() to the exit condition of cpu_idle_poll()
> to avoid this.
> 
> Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

> ---
> Changes from V1: Modified the Changelog
> 
>  kernel/sched/idle.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index c47fce7..aaf1c1d 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -47,7 +47,8 @@ static inline int cpu_idle_poll(void)
>  	rcu_idle_enter();
>  	trace_cpu_idle_rcuidle(0, smp_processor_id());
>  	local_irq_enable();
> -	while (!tif_need_resched())
> +	while (!tif_need_resched() &&
> +		(cpu_idle_force_poll || tick_check_broadcast_expired()))
>  		cpu_relax();
>  	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
>  	rcu_idle_exit();
> 
> 
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