From: Tejun Heo tj@kernel.org

[ Upstream commit e9fe182772dcb2630964724fd93e9c90b68ea0fd ]

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 Signed-off-by: Sasha Levin sashal@kernel.org --- tools/testing/selftests/sched_ext/dsp_local_on.bpf.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

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 c9a2da0575a0f..eea06decb6f59 100644 --- 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);