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

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Message-ID: <izltvlkmxwk4txk7vz4td5ytx3p7itlmdqyawc75halhmt22dh@jt34fctgso2h>
Date: Mon, 5 Jan 2026 08:41:01 -0800
From: Breno Leitao <leitao@...ian.org>
To: Christian Loehle <christian.loehle@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kernel-team@...a.com
Subject: Re: [PATCH] cpuidle: menu: Remove incorrect unlikely() annotation

On Mon, Jan 05, 2026 at 03:17:05PM +0000, Christian Loehle wrote:
> 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@...ian.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@...ian.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.

Probably not. I am just running PROFILE_ANNOTATED_BRANCHES tests on some
production arm64 host and addressing those that are making wrong
assumptions.