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

[K Prateek Nayak]

Hello folks,

On 3/21/2026 2:29 PM, K Prateek Nayak wrote:
> So I managed to reproduce the crash and it is actually crashing at:
> 
>   last->next = first;
> 
> in build_sched_groups(). If I print the span befora nd after we do
> the *sd = { ... }, I see:
> 
>   [    0.056301] span before: 0
>   [    0.056559] span after:
>   [    0.056686] span double check:
> 
> double check does a cpumask_pr_args(sched_domain_span(sd)).
> This solves the crash on top of this patch:
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 79bab80af8f2..b347ae5d2786 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1693,6 +1693,8 @@ sd_init(struct sched_domain_topology_level *tl,
>  		.name			= tl->name,
>  	};
>  
> +	cpumask_and(sd_span, cpu_map, tl->mask(tl, cpu));
> +
>  	WARN_ONCE((sd->flags & (SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY)) ==
>  		  (SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY),
>  		  "CPU capacity asymmetry not supported on SMT\n");
> ---
> 
> And I see:
> 
>   [    0.056479] span before: 0
>   [    0.056749] span after: 0
>   [    0.056881] span double check: 0
> 
> 
> But since span[] is a variable array at the end of sched_domain struct,
> doing a *sd = { ... } shouldn't modify it since the size isn't known at
> compile time and the compiler will only overwrite the fixed fields.
> 
> Is there a compiler angle I'm missing here?

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 :-(

-- 
Thanks and Regards,
Prateek