linux-kernel - Re: [PATCH v2] PM: EM: Fix potential division-by-zero error in em_compute

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Message-ID: <tencent_002D68A7E02F76C9D218B090D6085F0E8C06@qq.com>
Date: Tue, 15 Apr 2025 09:58:01 +0800
From: Yaxiong Tian <iambestgod@...com>
To: Lukasz Luba <lukasz.luba@....com>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Yaxiong Tian <tianyaxiong@...inos.cn>, rafael@...nel.org
Subject: Re: [PATCH v2] PM: EM: Fix potential division-by-zero error in
 em_compute_costs()



在 2025/4/15 09:12, Yaxiong Tian 写道:
> 
> 
> 在 2025/4/14 16:08, Lukasz Luba 写道:
>> Hi Yaxiong,
>>
>> On 4/11/25 02:28, Yaxiong Tian 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().
>>>
>>> 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")
>>> Signed-off-by: Yaxiong Tian <tianyaxiong@...inos.cn>
>>> ---
>>>   kernel/power/energy_model.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
>>> index d9b7e2b38c7a..d1fa7e8787b5 100644
>>> --- a/kernel/power/energy_model.c
>>> +++ b/kernel/power/energy_model.c
>>> @@ -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)) {
>>>               /* increase resolution of 'cost' precision */
>>>               power_res = table[i].power * 10;
>>>               cost = power_res / table[i].performance;
>>
>>
>> As the test robot pointed out, please set the 'cost' to 0
>> where it's declared.
>>
>> The rest should be fine.
>>
>> Regards,
>> Lukasz
> 
> Sorry, the V3 version with cost=0 still has issues.
> 
> I noticed that if the cost is set to 0, the condition "if (table[i].cost
>   >= prev_cost)" in the following code will always evaluate to true. This
>   will incorrectly set the flags to EM_PERF_STATE_INEFFICIENT.
> 
> Should we change ">=" to ">"?
> 

Sorry Again， Setting EM_PERF_STATE_INEFFICIENT in this case is correct.
Earlier, I misunderstood the definition/usage of EM_PERF_STATE_INEFFICIENT.