Re: [PATCH] cpuidle: menu: Remove incorrect unlikely() annotation

[Christian Loehle <christian.loehle@arm.com>]

On 1/5/26 14:37, Breno Leitao wrote:
> The unlikely() annotation on the early-return condition in menu_select()
> is incorrect on systems with only one idle state (e.g., ARM64 servers
> with a single ACPI LPI state). Branch profiling shows 100% misprediction
> on such systems since drv->state_count <= 1 is always true.
> 
> On platforms where only state0 is available, this path is the common
> case, not an unlikely edge case. Remove the misleading annotation to
> let the branch predictor learn the actual behavior.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  drivers/cpuidle/governors/menu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> index 64d6f7a1c776..ef9c5a84643e 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -271,7 +271,7 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>  		data->bucket = BUCKETS - 1;
>  	}
>  
> -	if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
> +	if (drv->state_count <= 1 || latency_req == 0 ||
>  	    ((data->next_timer_ns < drv->states[1].target_residency_ns ||
>  	      latency_req < drv->states[1].exit_latency_ns) &&
>  	     !dev->states_usage[0].disable)) {
> 
> ---
> base-commit: 34aa263125b6732375abcb908d73d98169154bb5
> change-id: 20260105-annotated_idle-d6b614ecd207
> 
> Best regards,
> --  
> Breno Leitao <leitao@debian.org>
> 
> 

Fine with me per se, I don't think the unlikely() annotation makes a
difference for the 'good case' either, but if you run into this I'd be curious
if you can see a difference with menu (which should stop the tick on every idle enter
regardless) and teo (which should never stop the tick on state_count == 1).
Alternative you can also just change the menu branch to not stop the tick.
I'd like to know if we need something more sophisticated generally here.