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

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Message-ID: <tencent_AE7CE43CDAD6747AFF0E952C8D0908A98D0A@qq.com>
Date: Tue, 15 Apr 2025 09:12:51 +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 2025/4/14 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 ">"?