linux-kernel - Re: [RFC PATCH 1/3] cpuidle: ladder: Fix bogus comparison between s64 and u64

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Message-ID: <CAJZ5v0gK60=Cya+u_-7qgL5LG5O9j=r+QH+Kwg71VQSGby6=YA@mail.gmail.com>
Date:   Wed, 23 Nov 2022 18:37:08 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Zhang Rui <rui.zhang@...el.com>
Cc:     rjw@...ysocki.net, daniel.lezcano@...aro.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] cpuidle: ladder: Fix bogus comparison between s64
 and u64

On Sat, Nov 5, 2022 at 6:40 PM Zhang Rui <rui.zhang@...el.com> wrote:
>
> ladder_device_state.threshold.promotion_time_ns/demotion_time_ns
> are u64 type.
>
> In ladder_select_state(), variable 'last_residency', as calculated by
>
> last_residency = dev->last_residency_ns - drv->states[last_idx].exit_latency_ns
>
> are s64 type, and it can be negative value.

The code changes are fine AFAICS, but the description below could be
more precise.

> When this happens, comparing between 'last_residency' and
> 'promotion_time_ns/demotion_time_ns' become bogus.

IIUC, what happens is that last_residency is converted to u64 in the
comparison expression and that conversion causes it to become a large
positive number if it is negative.

> As a result, the ladder governor promotes or stays with current state errornously.

"promotes or retains the current state erroneously".

>
>           <idle>-0       [001] d..1.   151.893396: ladder_select_state: last_idx 7, last_residency -373033
>           <idle>-0       [001] d..1.   151.893399: ladder_select_state:    dev->last_residency_ns 106967, drv->states[last_idx].exit_latency_ns 480000
>           <idle>-0       [001] d..1.   151.893402: ladder_select_state:    promote, last_state->threshold.promotion_time_ns 480000
>           <idle>-0       [001] d..1.   151.893404: ladder_select_state:    ---> new state 7
>           <idle>-0       [001] d..1.   151.893465: ladder_select_state: last_idx 7, last_residency -463800
>           <idle>-0       [001] d..1.   151.893467: ladder_select_state:    dev->last_residency_ns 16200, drv->states[last_idx].exit_latency_ns 480000
>           <idle>-0       [001] d..1.   151.893468: ladder_select_state:    promote, last_state->threshold.promotion_time_ns 480000
>           <idle>-0       [001] dn.1.   151.893470: ladder_select_state:    ---> new state 8
>
> Given that promotion_time_ns/demotion_time_ns are initialized with
> cpuidle_state.exit_latency_ns, which is s64 type, and they are used to
> compare with 'last_residency', which is also s64 type, there is no

"they are compared with"

> reason to use u64 for promotion_time_ns/demotion_time_ns.

"so change them both to be s64".

> With this patch,
>           <idle>-0       [001] d..1.   523.578531: ladder_select_state: last_idx 8, last_residency -879453
>           <idle>-0       [001] d..1.   523.578531: ladder_select_state:    dev->last_residency_ns 10547, drv->states[last_idx].exit_latency_ns 890000
>           <idle>-0       [001] d..1.   523.578532: ladder_select_state:    demote , last_state->threshold.demotion_time_ns 890000
>           <idle>-0       [001] d..1.   523.578532: ladder_select_state:    ---> new state 7
>           <idle>-0       [001] d..1.   523.580220: ladder_select_state: last_idx 7, last_residency -169629
>           <idle>-0       [001] d..1.   523.580221: ladder_select_state:    dev->last_residency_ns 310371, drv->states[last_idx].exit_latency_ns 480000
>           <idle>-0       [001] d..1.   523.580221: ladder_select_state:    demote , last_state->threshold.demotion_time_ns 480000
>           <idle>-0       [001] d..1.   523.580222: ladder_select_state:    ---> new state 6
>
> Signed-off-by: Zhang Rui <rui.zhang@...el.com>
> ---
>  drivers/cpuidle/governors/ladder.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/cpuidle/governors/ladder.c b/drivers/cpuidle/governors/ladder.c
> index 8e9058c4ea63..fb61118aef37 100644
> --- a/drivers/cpuidle/governors/ladder.c
> +++ b/drivers/cpuidle/governors/ladder.c
> @@ -27,8 +27,8 @@ struct ladder_device_state {
>         struct {
>                 u32 promotion_count;
>                 u32 demotion_count;
> -               u64 promotion_time_ns;
> -               u64 demotion_time_ns;
> +               s64 promotion_time_ns;
> +               s64 demotion_time_ns;
>         } threshold;
>         struct {
>                 int promotion_count;
> --