linux-kernel - Re: [RFC][PATCH 7/8] sched,idle: Delay clearing the polling bit

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Date:	Sun, 13 Apr 2014 17:51:03 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	mingo@...nel.org, tglx@...utronix.de, luto@...capital.net,
	daniel.lezcano@...aro.org, umgwanakikbuti@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 7/8] sched,idle: Delay clearing the polling bit

On Fri, 11 Apr 2014, Peter Zijlstra wrote:

> With the generic idle functions assuming !polling we should only clear
> the polling bit at the very last opportunity in order to avoid
> spurious IPIs.
> 
> Ideally we'd flip the default to polling, but that means auditing all
> arch idle functions.
> 
> Cc: Nicolas Pitre <nicolas.pitre@...aro.org>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>

Acked-by: Nicolas Pitre <nico@...aro.org>


> ---
>  kernel/sched/idle.c |   17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -78,12 +78,10 @@ static int cpuidle_idle_call(void)
>  
>  	/*
>  	 * Check if the idle task must be rescheduled. If it is the
> -	 * case, exit the function after re-enabling the local irq and
> -	 * set again the polling flag
> +	 * case, exit the function after re-enabling the local irq.
>  	 */
> -	if (current_clr_polling_and_test()) {
> +	if (need_resched()) {
>  		local_irq_enable();
> -		__current_set_polling();
>  		return 0;
>  	}
>  
> @@ -127,7 +125,7 @@ static int cpuidle_idle_call(void)
>  			broadcast = !!(drv->states[next_state].flags &
>  				       CPUIDLE_FLAG_TIMER_STOP);
>  
> -			if (broadcast)
> +			if (broadcast) {
>  				/*
>  				 * Tell the time framework to switch
>  				 * to a broadcast timer because our
> @@ -139,6 +137,7 @@ static int cpuidle_idle_call(void)
>  				ret = clockevents_notify(
>  					CLOCK_EVT_NOTIFY_BROADCAST_ENTER,
>  					&dev->cpu);
> +			}
>  
>  			if (!ret) {
>  				trace_cpu_idle_rcuidle(next_state, dev->cpu);
> @@ -175,8 +174,12 @@ static int cpuidle_idle_call(void)
>  	 * We can't use the cpuidle framework, let's use the default
>  	 * idle routine
>  	 */
> -	if (ret)
> -		arch_cpu_idle();
> +	if (ret) {
> +		if (!current_clr_polling_and_test())
> +			arch_cpu_idle();
> +		else
> +			local_irq_enable();
> +	}
>  
>  	__current_set_polling();
>  
> 
> 
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