linux-kernel - Re: [PATCH] sched/fair: fix task_numa_migrate to consider both task and group benefits

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Message-ID: <5b97da51-fc3e-4584-92e8-404e64c91101@gmail.com>
Date: Fri, 5 Sep 2025 11:01:22 +0800
From: Ethan Zhao <etzhao1900@...il.com>
To: Jianyong Wu <wujianyong@...on.cn>
Cc: "jianyong.wu@...look.com" <jianyong.wu@...look.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: fix task_numa_migrate to consider both task
 and group benefits



On 9/5/2025 10:14 AM, Jianyong Wu wrote:
> Hello Ethan,
> 
> Thanks for reply.
> 
> There is inconsistency between the comments and the code. See the discussion in an older patch here https://lkml.org/lkml/2015/6/16/540
The literal meaning of the comment appears somewhat misleading,
we always need to understand it combined with code context. or
needs more wording effort.

Thanks,
Ethan>
> Following that, the issue is with the comments, not the code.
> 
> Bests
> Jianyong
> 
>> -----Original Message-----
>> From: Ethan Zhao <etzhao1900@...il.com>
>> Sent: Friday, September 5, 2025 9:13 AM
>> To: Jianyong Wu <wujianyong@...on.cn>
>> Cc: jianyong.wu@...look.com; linux-kernel@...r.kernel.org
>> Subject: Re: [PATCH] sched/fair: fix task_numa_migrate to consider both task
>> and group benefits
>>
>>
>>
>> On 8/29/2025 4:55 PM, Jianyong Wu wrote:
>>> The comment indicates that when searching for a suitable NUMA node, we
>>> should ensure that the selected node benefits both the task and its
>>> NUMA group. However, the current implementation can only guarantee
>>> that either the task or the group benefits, but not necessarily both.
>>>
>>> Signed-off-by: Jianyong Wu <wujianyong@...on.cn>
>>> ---
>>>    kernel/sched/fair.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index
>>> b173a059315c..58c899738399 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -2568,7 +2568,7 @@ static int task_numa_migrate(struct task_struct
>> *p)
>>>    			/* Only consider nodes where both task and groups benefit
>> */
>>>    			taskimp = task_weight(p, nid, dist) - taskweight;
>>>    			groupimp = group_weight(p, nid, dist) - groupweight;
>>> -			if (taskimp < 0 && groupimp < 0)
>>> +			if (taskimp < 0 || groupimp < 0)
>> Perhaps you misunderstand the comment, && means either the task or the
>> group has NO benefit from this migration, it wouldn't be done.
>> But if you replace it with ||, you will ignore the target node that could benefit
>> either the task or the group.
>>
>> There is more logic to consider the benefit for both task & group in the later
>> function part.
>>
>> One question, why not
>> if (taskimp <= 0 && groupimp <= 0) ?
>>
>> Thanks,
>> Ethan
>>>    				continue;
>>>
>>>    			env.dist = dist;
>>
>