linux-kernel - Re: [PATCH 2/2] sched/idle: Use spin loop primitives for polling idle

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Date:   Fri, 1 Sep 2017 14:24:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/idle: Use spin loop primitives for polling idle

On Sun, Aug 20, 2017 at 07:25:02PM +1000, Nicholas Piggin wrote:
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
> ---
>  kernel/sched/idle.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 6c23e30c0e5c..b884980da8ef 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -11,6 +11,7 @@
>  #include <linux/stackprotector.h>
>  #include <linux/suspend.h>
>  #include <linux/livepatch.h>
> +#include <linux/processor.h>
>  
>  #include <asm/tlb.h>
>  
> @@ -64,9 +65,13 @@ static noinline int __cpuidle cpu_idle_poll(void)
>  	trace_cpu_idle_rcuidle(0, smp_processor_id());
>  	local_irq_enable();
>  	stop_critical_timings();
> +
> +	spin_begin();
>  	while (!tif_need_resched() &&
>  		(cpu_idle_force_poll || tick_check_broadcast_expired()))
> -		cpu_relax();
> +		spin_cpu_relax();
> +	spin_end();

Do we want at least one tif_need_resched() check before we drop into low
prio mode?

> +
>  	start_critical_timings();
>  	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
>  	rcu_idle_exit();
> -- 
> 2.13.3
>