linux-kernel - Re: [PATCH] idle/tick-broadcast: Exit cpu idle poll loop when cleared from tick_broadcast_force

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Message-ID: <alpine.DEB.2.11.1501201216420.5526@nanos>
Date:	Tue, 20 Jan 2015 12:21:24 +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] idle/tick-broadcast: Exit cpu idle poll loop when cleared
 from tick_broadcast_force_mask

On Mon, 19 Jan 2015, Preeti U Murthy wrote:
> An idle cpu enters cpu_idle_poll() if it is set in the tick_broadcast_force_mask.
> This is so that it does not incur the overhead of entering idle states when it is expected
> to be woken up anytime then through a broadcast IPI. The condition that forces an exit out
> of the idle polling is the check on setting of the TIF_NEED_RESCHED flag for the idle thread.
> 
> When the broadcast IPI does arrive, it is not guarenteed that the handler sets the
> TIF_NEED_RESCHED flag. Hence although the cpu is cleared in the tick_broadcast_force_mask,
> it continues to loop in cpu_idle_poll unnecessarily wasting power. Hence exit the idle
> poll loop if the tick_broadcast_force_mask gets cleared and enter idle states.
> 
> Of course if the cpu has entered cpu_idle_poll() on being asked to poll explicitly,
> it continues to poll till it is asked to reschedule.
> 
> Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> ---
> 
>  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()))

You explain the tick_check_broadcast_expired() bit, but what about the
cpu_idle_force_poll part?

Thanks,

	tglx
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