Re: bpf/selftests: test_access_variable_array breaks due to sched_domain::span removal

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

On Tue, Apr 07, 2026 at 10:33:51AM -0700, Alexei Starovoitov wrote:
> On Tue, Apr 7, 2026 at 9:12 AM Venkat Rao Bagalkote
> <venkat88@linux.ibm.com> wrote:
> >
> > Hi,
> >
> >
> > While running BPF selftests on current linux-next, I noticed that
> > test_access_variable_array fails to build due to reliance on
> > struct sched_domain::span, which is no longer appers to be BTF-visible
> > after recent
> > scheduler refactoring.
> >
> > The Build error I am seeing is:
> >
> > progs/test_access_variable_array.c:14:13: error: no member named 'span'
> > in 'struct sched_domain'  CLNG-BPF [test_progs] test_check_mtu.bpf.o
> >
> >     14 |         span = sd->span[0];
> >        |                ~~  ^
> >
> > Below is a proposed update to the test that switches from
> > sched_domain::span to sched_group::cpumask. This preserves the original
> > intent of validating variable-length array access via BTF while avoiding
> > reliance on removed scheduler internals.
> >
> >
> > diff --git
> > a/tools/testing/selftests/bpf/progs/test_access_variable_array.c
> > b/tools/testing/selftests/bpf/progs/test_access_variable_array.c
> > index 326b7d1f496a..c9f345ccde3c 100644
> > --- a/tools/testing/selftests/bpf/progs/test_access_variable_array.c
> > +++ b/tools/testing/selftests/bpf/progs/test_access_variable_array.c
> > @@ -4,14 +4,18 @@
> >   #include "vmlinux.h"
> >   #include <bpf/bpf_helpers.h>
> >   #include <bpf/bpf_tracing.h>
> > +#include <bpf/bpf_core_read.h>
> >
> > -unsigned long span = 0;
> > +unsigned long cpumask0 = 0;
> >
> > -SEC("fentry/sched_balance_rq")
> > -int BPF_PROG(fentry_fentry, int this_cpu, struct rq *this_rq,
> > -               struct sched_domain *sd)
> > +SEC("fentry/sched_balance_find_dst_group_cpu")
> > +int BPF_PROG(fentry_fentry, struct sched_group *sg, struct task_struct *p,
> > +               int this_cpu)
> >   {
> > -       span = sd->span[0];
> > +       unsigned long *mask;
> > +       /* Read pointer to variable-length CPU mask */
> > +       mask = BPF_CORE_READ(sg, cpumask);
> > +       cpumask0 = mask[0];
> 
> No. That defeats the point of the test.
> It tests [] array in the kernel struct.
> Just make one in testmod.ko and access that.
Hi Alexei,

We do have cpumask as [] array in sched_group:

struct sched_group {
	struct sched_group	*next;			/* Must be a circular list */
	atomic_t		ref;

	unsigned int		group_weight;
	unsigned int		cores;
	struct sched_group_capacity *sgc;
	int			asym_prefer_cpu;	/* CPU of highest priority in group */
	int			flags;

	/*
	 * The CPUs this group covers.
	 *
	 * NOTE: this field is variable length. (Allocated dynamically
	 * by attaching extra space to the end of the structure,
	 * depending on how many CPUs the kernel has booted up with)
	 */
	unsigned long		cpumask[];
};

Wouldn't be good if it is used as:

cpumask0 = sg->cpumask[0];

Also, there is already a selftest that makes [] in testmod.ko
and access that.

Thanks,
Saket