linux-kernel - Re: [PATCH v4] PM: EM: Fix potential division-by-zero error in em_compute

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Message-ID: <tencent_91D93A837FF94A06C7FE8492461472B86D09@qq.com>
Date: Thu, 17 Apr 2025 15:43:27 +0800
From: Yaxiong Tian <iambestgod@...com>
To: Lukasz Luba <lukasz.luba@....com>, rafael@...nel.org
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Yaxiong Tian <tianyaxiong@...inos.cn>
Subject: Re: [PATCH v4] PM: EM: Fix potential division-by-zero error in
 em_compute_costs()



在 2025/4/17 13:57, Lukasz Luba 写道:
> 
> 
> On 4/17/25 02:07, 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 | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
>> index d9b7e2b38c7a..41606247c277 100644
>> --- a/kernel/power/energy_model.c
>> +++ b/kernel/power/energy_model.c
>> @@ -233,6 +233,10 @@ static int em_compute_costs(struct device *dev, 
>> struct em_perf_state *table,
>>       unsigned long prev_cost = ULONG_MAX;
>>       int i, ret;
>> +    /* This is needed only for CPUs and EAS skip other devices */
>> +    if (!_is_cpu_device(dev))
>> +        return 0;
>> +
>>       /* Compute the cost of each performance state. */
>>       for (i = nr_states - 1; i >= 0; i--) {
>>           unsigned long power_res, cost;
> 
> 
> Please stop for a while. I have to check what happened that you
> faced the issue in the first place. I have been testing the GPU
> EMs and there was no issues...
> 
> Let me debug that today.

Of course. Since I don't have actual hardware, I can only logically
deduce that this issue might exist.