linux-kernel - Re: [PATCH 5/6] cpuidle: menu: Avoid computations for very close timers

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Date:   Thu, 4 Oct 2018 17:50:44 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] cpuidle: menu: Avoid computations for very close
 timers

On Tue, Oct 02, 2018 at 11:46:28PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> If the next timer event (with the tick excluded) is closer than the
> target residency of the second state or the PM QoS latency constraint
> is below its exit latency, state[0] will be used regardless of any
> other factors, so skip the computations in menu_select() then and
> return 0 straight away from it.
> 
> Still, do that after the bucket has been determined to avoid
> disturbing the wakeup statistics in general.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpuidle/governors/menu.c |   12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> Index: linux-pm/drivers/cpuidle/governors/menu.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/governors/menu.c
> +++ linux-pm/drivers/cpuidle/governors/menu.c
> @@ -309,6 +309,18 @@ static int menu_select(struct cpuidle_dr
>  	get_iowait_load(&nr_iowaiters, &cpu_load);
>  	data->bucket = which_bucket(data->next_timer_us, nr_iowaiters);
>  
> +	if (unlikely(drv->state_count <= 1) ||

I'm not sure this test is necessary.

If state_count == 0, we don't have to select anything as the cpuidle can't register because of:

static int __cpuidle_register_driver(struct cpuidle_driver *drv)
{
        int ret;

        if (!drv || !drv->state_count)
                return -EINVAL;

	[ ... ]
}

If state_count == 1, then there is nothing to select, it is always the state 0.

> +	    ((data->next_timer_us < drv->states[1].target_residency ||
> +	      latency_req < drv->states[1].exit_latency) &&
> +	     !drv->states[0].disabled && !dev->states_usage[0].disable)) {
> +		/*
> +		 * In this case state[0] will be used no matter what, so return
> +		 * it right away and keep the tick running.
> +		 */
> +		*stop_tick = false;
> +		return 0;
> +	}
> +
>  	/*
>  	 * Force the result of multiplication to be 64 bits even if both
>  	 * operands are 32 bits.
> 

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