sched_ext: selftests/dsp_local_on: Fix sporadic failures

dsp_local_on has several incorrect assumptions, one of which is that
p->nr_cpus_allowed always tracks p->cpus_ptr. This is not true when a task
is scheduled out while migration is disabled - p->cpus_ptr is temporarily
overridden to the previous CPU while p->nr_cpus_allowed remains unchanged.

This led to sporadic test faliures when dsp_local_on_dispatch() tries to put
a migration disabled task to a different CPU. Fix it by keeping the previous
CPU when migration is disabled.

There are SCX schedulers that make use of p->nr_cpus_allowed. They should
also implement explicit handling for p->migration_disabled.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Ihor Solodrai <ihor.solodrai@pm.me>
Cc: Andrea Righi <arighi@nvidia.com>
Cc: Changwoo Min <changwoo@igalia.com>

diff --git a/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c b/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c
index fbda6bf5467128810bf14cf0da6255eb507d8b7d..758b479bd1ee17bfec0c8cedcf842bfb71eacaea 100644 (file)
--- a/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c
+++ b/tools/testing/selftests/sched_ext/dsp_local_on.bpf.c
@@ -43,7 +43,7 @@ void BPF_STRUCT_OPS(dsp_local_on_dispatch, s32 cpu, struct task_struct *prev)
        if (!p)
                return;
 
-       if (p->nr_cpus_allowed == nr_cpus)
+       if (p->nr_cpus_allowed == nr_cpus && !p->migration_disabled)
                target = bpf_get_prandom_u32() % nr_cpus;
        else
                target = scx_bpf_task_cpu(p);