sched_ext: idle: Handle migration-disabled tasks in idle selection

When SCX_OPS_ENQ_MIGRATION_DISABLED is enabled, migration-disabled tasks
are also routed to ops.enqueue(). A scheduler may attempt to dispatch
such tasks directly to an idle CPU using the default idle selection
policy via scx_bpf_select_cpu_and() or scx_bpf_select_cpu_dfl().

This scenario must be properly handled by the built-in idle policy to
avoid returning an idle CPU where the target task isn't allowed to run.
Otherwise, it can lead to errors such as:

 EXIT: runtime error (SCX_DSQ_LOCAL[_ON] cannot move migration disabled Chrome_ChildIOT[291646] from CPU 3 to 14)

Prevent this by explicitly handling migration-disabled tasks in the
built-in idle selection logic, maintaining their CPU affinity.

Fixes: a730e3f7a48bc ("sched_ext: idle: Consolidate default idle CPU selection kfuncs")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index 6d29d3cbc6707c1630ab3e1cee5a529c0d9fe56f..001fb88a8481d892949cb9e26ec200e995f15fd6 100644 (file)
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -903,7 +903,7 @@ s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
         * selection optimizations and simply check whether the previously
         * used CPU is idle and within the allowed cpumask.
         */
-       if (p->nr_cpus_allowed == 1) {
+       if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
                if (cpumask_test_cpu(prev_cpu, allowed ?: p->cpus_ptr) &&
                    scx_idle_test_and_clear_cpu(prev_cpu))
                        cpu = prev_cpu;