linux-kernel - Re: [PATCH] PM: EM: Fix potential division-by-zero error in em_compute

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Message-ID: <CAJZ5v0jfAdBbKXBg7k0og6MucptJc9G=RTzFPd=N3Q0VdfToFQ@mail.gmail.com>
Date: Thu, 10 Apr 2025 15:23:59 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Yaxiong Tian <iambestgod@...com>
Cc: lukasz.luba@....com, rafael@...nel.org, len.brown@...el.com, 
	pavel@...nel.org, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Yaxiong Tian <tianyaxiong@...inos.cn>
Subject: Re: [PATCH] PM: EM: Fix potential division-by-zero error in em_compute_costs()

On Thu, Apr 10, 2025 at 7:39 AM Yaxiong Tian <iambestgod@...com> wrote:
>
> From: Yaxiong Tian <tianyaxiong@...inos.cn>
>
> When the device is of a non-CPU type, table[i].performance won't be
> initialized in the previous em_init_performance(), resulting in division
>  by zero when calculating costs in em_compute_costs().
>
> Considering that the performance field in struct em_perf_state is defined
> as "CPU performance (capacity) at a given frequency", the original
> calculation method should be maintained when the device is of a non-CPU
> type.
>
> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
>
> Signed-off-by: Yaxiong Tian <tianyaxiong@...inos.cn>
> ---
>  kernel/power/energy_model.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index d9b7e2b38c7a..bbd95573d91e 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -231,9 +231,11 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>                             unsigned long flags)
>  {
>         unsigned long prev_cost = ULONG_MAX;
> +       u64 fmax;

Why not initialize it here?  Also please retain the reverse x-mas tree
ordering of declarations.

>         int i, ret;
>
>         /* Compute the cost of each performance state. */
> +       fmax = (u64) table[nr_states - 1].frequency;

No need to cast to u64 explicitly (it will be cast anyway).

>         for (i = nr_states - 1; i >= 0; i--) {
>                 unsigned long power_res, cost;
>
> @@ -245,9 +247,15 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>                                 return -EINVAL;
>                         }
>                 } else {
> -                       /* increase resolution of 'cost' precision */
> -                       power_res = table[i].power * 10;
> -                       cost = power_res / table[i].performance;
> +                       if (_is_cpu_device(dev)) {
> +                               /* increase resolution of 'cost' precision */
> +                               power_res = table[i].power * 10;
> +                               cost = power_res / table[i].performance;
> +                       } else {
> +                               power_res = table[i].power;
> +                               cost = div64_u64(fmax * power_res, table[i].frequency);

Why is it necessary to compute the "cost" field value for non-CPU
devices at all?

> +

An excess empty line.

> +                       }
>                 }
>
>                 table[i].cost = cost;
> --