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

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Message-ID: <6446b46bb8750deb28687b3e84d9b6062a35c86a.camel@linux.intel.com>
Date: Tue, 30 Sep 2025 10:30:36 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: "Chen, Yu C" <yu.c.chen@...el.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 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.

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),