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

JIRA: https://issues.redhat.com/browse/RHEL-111810

commit 06efc9f
Author: Andrea Righi <arighi@nvidia.com>
Date:   Sat Jul 5 07:43:51 2025 +0200

    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: a730e3f ("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>

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/sched/ext_idle.c

@@ -898,7 +898,7 @@ static s32 select_cpu_from_kfunc(struct task_struct *p, s32 prev_cpu, u64 wake_f
 	 * 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;