Re: [PATCH] cpuidle: menu: Retain tick when shallow state is selected

[leo.yan@linaro.org]

On Tue, Aug 21, 2018 at 10:44:10AM +0200, Rafael J . Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> The case addressed by commit 5ef499cd571c (cpuidle: menu: Handle
> stopped tick more aggressively) in the stopped tick case is present
> when the tick has not been stopped yet too.  Namely, if only two CPU
> idle states, shallow state A with target residency significantly
> below the tick boundary and deep state B with target residency
> significantly above it, are available and the predicted idle
> duration is above the tick boundary, but below the target residency
> of state B, state A will be selected and the CPU may spend indefinite
> amount of time in it, which is not quite energy-efficient.
> 
> However, if the tick has not been stopped yet and the governor is
> about to select a shallow idle state for the CPU even though the idle
> duration predicted by it is above the tick boundary, it should be
> fine to wake up the CPU early, so the tick can be retained then and
> the governor will have a chance to select a deeper state when it runs
> next time.
> 
> [Note that when this really happens, it will make the idle duration
>  predictor believe that the CPU might be idle longer than predicted,
>  which will make it more likely to predict longer idle durations going
>  forward, but that will also cause deeper idle states to be selected
>  going forward, on average, which is what's needed here.]
> 
> Fixes: 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states with stopped tick)
> Reported-by: Leo Yan <leo.yan@linaro.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> 
> Commit 5ef499cd571c (cpuidle: menu: Handle stopped tick more aggressively) is
> in linux-next only at this point.
> 
> ---
>  drivers/cpuidle/governors/menu.c |   13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> Index: linux-pm/drivers/cpuidle/governors/menu.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/governors/menu.c
> +++ linux-pm/drivers/cpuidle/governors/menu.c
> @@ -379,9 +379,20 @@ static int menu_select(struct cpuidle_dr
>  		if (idx == -1)
>  			idx = i; /* first enabled state */
>  		if (s->target_residency > data->predicted_us) {
> -			if (!tick_nohz_tick_stopped())
> +			if (data->predicted_us < TICK_USEC)

With this change, for tick has been stopped, it might introduce
regression to select a shallow state and it's conflict with the effect
of patch "cpuidle: menu: Handle stopped tick more aggressively".

>  				break;
>  
> +			if (!tick_nohz_tick_stopped()) {
> +				/*
> +				 * If the state selected so far is shallow,
> +				 * waking up early won't hurt, so retain the
> +				 * tick in that case and let the governor run
> +				 * again in the next iteration of the loop.
> +				 */
> +				expected_interval = drv->states[idx].target_residency;
> +				break;
> +			}
> +

This is reliable, how we can rely on a shallow idle state target
residency to decide if need to stop a tick or not?

>  			/*
>  			 * If the state selected so far is shallow and this
>  			 * state's target residency matches the time till the
>