linux-kernel - Re: [PATCH] cpuidle: teo: fix underflow of recent intercepts

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Date: Mon, 20 May 2024 10:59:45 +0100
From: Christian Loehle <christian.loehle@....com>
To: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Qais Yousef <qyousef@...alina.io>, kajetan.puchalski@....com,
 Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [PATCH] cpuidle: teo: fix underflow of recent intercepts

On 5/19/24 22:03, Christian Loehle wrote:
> The recent counter of each cpuidle state bin reflects the number of
> recent intercepts. It's decremented and incremented accordingly.
> The decrement was never checked for 0, therefore underflowing into a
> value teo cannot easily recover from.
> 
> The underflow lead to deeper idle states being skipped because teo
> assumed interception was likely and it preferring shallower states.
> 
> Fixes: 77577558f25d ("cpuidle: teo: Rework most recent idle duration values treatment")
> Signed-off-by: Christian Loehle <christian.loehle@....com>
> ---
>  drivers/cpuidle/governors/teo.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpuidle/governors/teo.c b/drivers/cpuidle/governors/teo.c
> index 7244f71c59c5..42fb2771e35d 100644
> --- a/drivers/cpuidle/governors/teo.c
> +++ b/drivers/cpuidle/governors/teo.c
> @@ -290,7 +290,8 @@ static void teo_update(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>  	if (cpu_data->next_recent_idx >= NR_RECENT)
>  		cpu_data->next_recent_idx = 0;
>  
> -	if (cpu_data->recent_idx[i] >= 0)
> +	if (cpu_data->recent_idx[i] >= 0 &&
> +			cpu_data->state_bins[cpu_data->recent_idx[i]].recent)
>  		cpu_data->state_bins[cpu_data->recent_idx[i]].recent--;
>  
>  	/*

You should be able to observe the problematic underflow on any system.
Essentially these lines:
	alt_intercepts = 2 * idx_intercept_sum > cpu_data->total - idx_hit_sum;
	alt_recent = idx_recent_sum > NR_RECENT / 2; // CL: idx_recent_sum being the sum of a bunch of underflowed recent values, so expected to be big, too.
	if (alt_recent || alt_intercepts) {
become
	if (true) {

but the if block actually overriding the state should only be possible
for systems with >2 cpuidle states.
I'll follow up with a more detailed report though.