Re: [tip: sched/core] sched/topology: Compute sd_weight considering cpuset partitions

[K Prateek Nayak]

On 3/21/2026 3:15 PM, K Prateek Nayak wrote:
> So this is what I've found: By default we have:
> 
>   cpumask_size: 4
>   struct sched_domain size: 296
> 
> If I do:
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index a1e1032426dc..f0bebce274f7 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -148,7 +148,7 @@ struct sched_domain {
>  	 * by attaching extra space to the end of the structure,
>  	 * depending on how many CPUs the kernel has booted up with)
>  	 */
> -	unsigned long span[];
> +	unsigned long span[1];
>  };
>  
>  static inline struct cpumask *sched_domain_span(struct sched_domain *sd)
> ---
> 
> I still see:
> 
>   cpumask_size: 4
>   struct sched_domain size: 296
> 
> Which means we are overwriting the sd->span during *sd assignment even
> with the variable length array at the end :-(
> 

And more evidence - by default we have:

  sched_domain size: 296
  offset of sd_span: 292

sizeof() seems to account some sort of 4-byte padding for the struct which
pushes the offset of sd->span into the struct size.

To resolve this, we can also do:

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index a1e1032426dc..48bea2f7f750 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -148,7 +148,7 @@ struct sched_domain {
 	 * by attaching extra space to the end of the structure,
 	 * depending on how many CPUs the kernel has booted up with)
 	 */
-	unsigned long span[];
+	unsigned long span[] __aligned(2 * sizeof(int));
 };
 
 static inline struct cpumask *sched_domain_span(struct sched_domain *sd)
---

and the kernel boots fine with the sd_span offset aligned with
sched_domain struct size:

  sched_domain size: 296
  offset of sd_span: 296


So Peter, which solution do you prefer?

1. Doing cpumask_and() after the *sd = { ... } initialization. (or)

2. Align sd->span to an 8-byte boundary.

-- 
Thanks and Regards,
Prateek