Re: [PATCH v3] PM: EM: Fix potential division-by-zero error in em_compute_costs()

[Yaxiong Tian <iambestgod@qq.com>]

在 2025/4/16 19:58, Rafael J. Wysocki 写道:
> On Wed, Apr 16, 2025 at 4:57 AM Yaxiong Tian <iambestgod@qq.com> wrote:
>>
>> 在 2025/4/16 01:17, Rafael J. Wysocki 写道:
>>> On Mon, Apr 14, 2025 at 11:09 AM Yaxiong Tian <iambestgod@qq.com> wrote:
>>>>
>>>> From: Yaxiong Tian <tianyaxiong@kylinos.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().
>>>>
>>>> Since the 'cost' algorithm is only used for EAS energy efficiency
>>>> calculations and is currently not utilized by other device drivers, we
>>>> should add the _is_cpu_device(dev) check to prevent this division-by-zero
>>>> issue.
>>>>
>>>> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
>>>
>>> Please look at the Fixes: tags in the kernel git history.  They don't
>>> look like the one above.
>>>
>> Yes, there's an extra '<>' here.
>>
>>>> Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
>>>> ---
>>>>    kernel/power/energy_model.c | 4 ++--
>>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
>>>> index d9b7e2b38c7a..fc972cc1fc12 100644
>>>> --- a/kernel/power/energy_model.c
>>>> +++ b/kernel/power/energy_model.c
>>>> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>>>>
>>>>           /* Compute the cost of each performance state. */
>>>>           for (i = nr_states - 1; i >= 0; i--) {
>>>> -               unsigned long power_res, cost;
>>>> +               unsigned long power_res, cost = 0;
>>>>
>>>>                   if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
>>>>                           ret = cb->get_cost(dev, table[i].frequency, &cost);
>>>> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>>>>                                           cost, ret);
>>>>                                   return -EINVAL;
>>>>                           }
>>>> -               } else {
>>>> +               } else if (_is_cpu_device(dev)) {
>>>
>>> Can't you just check this upfront at the beginning of the function and
>>> make it bail out if dev is not a CPU device?
>>>
>> Sure, But the current implementation applies em_compute_costs() to both
>> non-CPU devices and CPU devices.
> 
> Maybe it shouldn't do that for non-CPU ones?
> 
>> After carefully reviewing the latest code,
>> I've found this issue has expanded in scope.
>>
>> There are currently three call paths for invoking em_compute_costs():
>>
>> 1) Registering performance domains (for both non-CPU and CPU devices)
>> em_dev_register_perf_domain() → em_create_pd() →
>> em_create_perf_table() → em_compute_costs()
>>
>> 2)EM update paths (CPU devices only)
>>
>> Periodic 1000ms update check via em_update_work work item:
>> em_check_capacity_update() → em_adjust_new_capacity() →
>> em_recalc_and_update() → em_compute_costs()
>>
>> Exynos-chip initialization:
>> em_dev_update_chip_binning() → em_recalc_and_update() → em_compute_costs()
>>
>> 3) Device cost computation (non-CPU devices only - currently unused)
>> em_dev_compute_costs() → em_compute_costs()
> 
> So because this one is unused and AFAICS the cost values are never
> used for non-CPU devices, it's better to just avoid computing them at
> all.
> 
>> Note: In em_dev_compute_costs(), when calling em_compute_costs(),
>> neither the callback (cb) nor flags are set.In fact, it either does
>> nothing at all or performs incorrect operations.
>>
>> Therefore, should we mandate that non-CPU devices must provide a
>> get_cost callback?
> 
> Why would that be an improvement?
> 
>> So Should we add a check at the beginning of the em_compute_costs() to:
>>
>>          if (!_is_cpu_device(dev) && !cb->get_cost) {
>>                  dev_dbg(dev, "EM: No get_cost provided, cost unset.\n");
>>                  return 0;
>>          }
>> And Modify em_dev_compute_costs() to require callers to provide the cb
>> callback function,Also need to update its corresponding comments.
>>
>>
>>>>                           /* increase resolution of 'cost' precision */
>>>>                           power_res = table[i].power * 10;
>>>>                           cost = power_res / table[i].performance;
>>>> --
> 
> I think until there is a user of em_dev_compute_costs() this is all
> moot and hard to figure out.
> 
> I would drop em_dev_compute_costs() altogether for now and put a
> _is_cpu_device(dev) upfront check into em_compute_costs().

Yes, I agree with your point. Currently no non-CPU devices are using
'cost'. The best approach would be to just add the _is_cpu_device check. 
I'll update it in V4.

By the way, em_dev_compute_costs should only apply to CPU devices. I was
mistaken earlier—it’s really hard to tell just from the function name.