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

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

[Venkat Rao Bagalkote]

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];

         return 0;
  }


I have tested this change, and it seems to be working as expected.


# ./test_progs -t access_variable_array
#1       access_variable_array:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED


Please note that this comes from my early days of working on upstream
kernel contributions, and I am still learning the BPF and scheduler
internals. My understanding here may be incomplete, so I wanted to
share this primarily to report the breakage I am seeing and propose a
possible direction for fixing the test.

I would appreciate any feedback on whether this is the right approach,
or if there is a more appropriate structure or hook to use for
preserving the variable-length array coverage in this selftest.


Regards,

Venkat.

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

[Alexei Starovoitov]

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.

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

[Saket Kumar Bhaskar]

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

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

[Peter Zijlstra]

On Wed, Apr 08, 2026 at 10:21:46AM +0530, Saket Kumar Bhaskar wrote:

> 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[];
> };

For now. But given how utterly broken flex arrays are, this thing will
be gone the second we run into that same thing again.

> Wouldn't be good if it is used as:
> 
> cpumask0 = sg->cpumask[0];

I'm thinking Alexei's point is that this test should not rely on random
kernel code.

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

[Alan Maguire]

On 08/04/2026 08:56, Peter Zijlstra wrote:
> On Wed, Apr 08, 2026 at 10:21:46AM +0530, Saket Kumar Bhaskar wrote:
> 
>> 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[];
>> };
> 
> For now. But given how utterly broken flex arrays are, this thing will
> be gone the second we run into that same thing again.
> 
>> Wouldn't be good if it is used as:
>>
>> cpumask0 = sg->cpumask[0];
> 
> I'm thinking Alexei's point is that this test should not rely on random
> kernel code.
> 

looks like we already have

struct bpf_testmod_struct_arg_3 {
        int a;
        int b[];
};

and function 

int bpf_testmod_test_struct_arg_3(int a, int b, struct bpf_testmod_struct_arg_2 c);

...in selftests/bpf/test_kmods/bpf_testmod.c so we could potentially use that instead.

Alan