linux-kernel - Re: [PATCH v4 1/2] sched: Create architecture specific sched domain distances

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Message-ID: <10a7e801-6cb5-4f2f-b72b-5f6733054fe1@intel.com>
Date: Wed, 1 Oct 2025 09:10:53 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>, Peter Zijlstra
	<peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>
CC: Juri Lelli <juri.lelli@...hat.com>, Dietmar Eggemann
	<dietmar.eggemann@....com>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, Tim Chen
	<tim.c.chen@...el.com>, Vincent Guittot <vincent.guittot@...aro.org>, "Len
 Brown" <len.brown@...el.com>, <linux-kernel@...r.kernel.org>, K Prateek Nayak
	<kprateek.nayak@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
	"Zhao Liu" <zhao1.liu@...el.com>, Vinicius Costa Gomes
	<vinicius.gomes@...el.com>, Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [PATCH v4 1/2] sched: Create architecture specific sched domain
 distances

On 10/1/2025 1:30 AM, Tim Chen wrote:
> On Tue, 2025-09-30 at 10:28 +0800, Chen, Yu C wrote:
>> On 9/30/2025 6:18 AM, Tim Chen wrote:
>>> On Sat, 2025-09-27 at 20:34 +0800, Chen, Yu C wrote:
>>>>
> 
> [snip]
> 
>>>>
>>>> If our goal is to figure out whether the arch_sched_node_distance()
>>>> has been overridden, how about the following alias?
>>>>
>>>> int __weak arch_sched_node_distance(int from, int to)
>>>> {
>>>> 	return __node_distance(from, to);
>>>> }
>>>> int arch_sched_node_distance_original(int from, int to) __weak
>>>> __alias(arch_sched_node_distance);
>>>>
>>>> static bool arch_sched_node_distance_is_overridden(void)
>>>> {
>>>> 	return arch_sched_node_distance != arch_sched_node_distance_original;
>>>> }
>>>>
>>>> so arch_sched_node_distance_is_overridden() can replace
>>>> modified_sched_node_distance()
>>>>
>>>
>>> I think that the alias version will still point to the replaced function and not
>>> the originally defined one.
>>>
>>> How about not using __weak and just explicitly define arch_sched_node_distance
>>> as a function pointer.  Change the code like below.
>>>
>>
>> The arch_sched_node_distance_original is defined as __weak, so it
>> should point to the old function even if the function has been
>> overridden. I did a test on a X86 VM and it seems to be so.
>> But using the arch_sched_node_distance as a function point
>> should also be OK.
>>
> 
> How about changing the code as follow. I think this change is cleaner.
> I tested it in my VM and works for detecting sched distance substitution.
> Thanks.
> 

Yes, the following change looks good to me.

Thanks,
Chenyu> Tim
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index f25e4402c63e..3dc941258df3 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1897,31 +1897,17 @@ static void init_numa_topology_type(int offline_node)
>    * A NUMA level is created for each unique
>    * arch_sched_node_distance.
>    */
> -static bool __modified_sched_node_dist = true;
> -
> -int __weak arch_sched_node_distance(int from, int to)
> +static int numa_node_dist(int i, int j)
>   {
> -	if (__modified_sched_node_dist)
> -		__modified_sched_node_dist = false;
> -
> -	return node_distance(from, to);
> +	return node_distance(i, j);
>   }
>   
> -static bool modified_sched_node_distance(void)
> -{
> -	/*
> -	 * Call arch_sched_node_distance()
> -	 * to determine if arch_sched_node_distance
> -	 * has been modified from node_distance()
> -	 * to arch specific distance.
> -	 */
> -	arch_sched_node_distance(0, 0);
> -	return __modified_sched_node_dist;
> -}
> +int arch_sched_node_distance(int from, int to)
> +			     __weak __alias(numa_node_dist);
>   
> -static int numa_node_dist(int i, int j)
> +static bool modified_sched_node_distance(void)
>   {
> -	return node_distance(i, j);
> +	return numa_node_dist != arch_sched_node_distance;
>   }
>   
>   static int sched_record_numa_dist(int offline_node, int (*n_dist)(int, int),
>