Subject: Re: [PATCH] cpuidle/menu: Fail cpuidle_idle_call() if no idle state is acceptable

From: "Srivatsa S. Bhat" <[email-masked]>
Date: Tue, 14 Jan 2014 12:30:14 +0530

On 01/14/2014 11:35 AM, Preeti U Murthy wrote:
> On PowerPC, in a particular test scenario, all the cpu idle states were disabled.
> Inspite of this it was observed that the idle state count of the shallowest
> idle state, snooze, was increasing.
> 
> This is because the governor returns the idle state index as 0 even in
> scenarios when no idle state can be chosen. These scenarios could be when the
> latency requirement is 0 or as mentioned above when the user wants to disable
> certain cpu idle states at runtime. In the latter case, its possible that no
> cpu idle state is valid because the suitable states were disabled
> and the rest did not match the menu governor criteria to be chosen as the
> next idle state.
> 
> This patch adds the code to indicate that a valid cpu idle state could not be
> chosen by the menu governor and reports back to arch so that it can take some
> default action.
> 

That sounds fair enough. However, the "default" action of pseries idle loop
(pseries_lpar_idle()) surprises me. It enters Cede, which is _deeper_ than doing
a snooze! IOW, a user might "disable" cpuidle or set the PM_QOS_CPU_DMA_LATENCY
to 0 hoping to prevent the CPUs from going to deep idle states, but then the
machine would still end up going to Cede, even though that wont get reflected
in the idle state counts. IMHO that scenario needs some thought as well...

> Signed-off-by: Preeti U Murthy <[email-masked]>
> ---
> 
>  drivers/cpuidle/cpuidle.c        |    6 +++++-
>  drivers/cpuidle/governors/menu.c |    7 ++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index a55e68f..5bf06bb 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -131,8 +131,9 @@ int cpuidle_idle_call(void)
> 
>  	/* ask the governor for the next state */
>  	next_state = cpuidle_curr_governor->select(drv, dev);
> +
> +	dev->last_residency = 0;
>  	if (need_resched()) {
> -		dev->last_residency = 0;
>  		/* give the governor an opportunity to reflect on the outcome */
>  		if (cpuidle_curr_governor->reflect)
>  			cpuidle_curr_governor->reflect(dev, next_state);

The comments on top of the .reflect() routines of the governors say that the
second parameter is the index of the actual state entered. But after this patch,
next_state can be negative, indicating an invalid index. So those comments need
to be updated accordingly.

> @@ -140,6 +141,9 @@ int cpuidle_idle_call(void)
>  		return 0;
>  	}
> 
> +	if (next_state < 0)
> +		return -EINVAL;

The exit path above (due to need_resched) returns with irqs enabled, but the new
one you are adding (next_state < 0) returns with irqs disabled. This is correct,
because in the latter case, "idle" is still in progress and the arch will choose
a default handler to execute (unlike the former case where "idle" is over and
hence its time to enable interrupts).

IMHO it would be good to add comments around this code to explain this subtle
difference. We can never be too careful with these things... ;-)

> +
>  	trace_cpu_idle_rcuidle(next_state, dev->cpu);
> 
>  	broadcast = !!(drv->states[next_state].flags & CPUIDLE_FLAG_TIMER_STOP);
> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> index cf7f2f0..6921543 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -283,6 +283,7 @@ again:
>   * menu_select - selects the next idle state to enter
>   * @drv: cpuidle driver containing state data
>   * @dev: the CPU
> + * Returns -1 when no idle state is suitable
>   */
>  static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>  {
> @@ -292,17 +293,17 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>  	int multiplier;
>  	struct timespec t;
> 
> -	if (data->needs_update) {
> +	if (data->last_state_idx >= 0 && data->needs_update) {
               ^^^^^
Doesn't hurt, but actually unnecessary, since ->needs_update is set to 1
only when index >= 0.

>  		menu_update(drv, dev);
>  		data->needs_update = 0;
>  	}
> 
> -	data->last_state_idx = 0;
> +	data->last_state_idx = -1;
>  	data->exit_us = 0;
> 
>  	/* Special case when user has set very strict latency requirement */
>  	if (unlikely(latency_req == 0))
> -		return 0;
> +		return data->last_state_idx;
> 
>  	/* determine the expected residency time, round up */
>  	t = ktime_to_timespec(tick_nohz_get_sleep_length());
> 

What about the ladder governor? I know its not used that much in practice,
but I think it would be good to update that as well, just to keep it
consistent.

Regards,
Srivatsa S. Bhat