sched: Make the idle task quack like a per-CPU kthread
For all intents and purposes, the idle task is a per-CPU kthread. It isn't
created via the same route as other pcpu kthreads however, and as a result
it is missing a few bells and whistles: it fails kthread_is_per_cpu() and
it doesn't have PF_NO_SETAFFINITY set.
Fix the former by giving the idle task a kthread struct along with the
KTHREAD_IS_PER_CPU flag. This requires some extra iffery as init_idle()
call be called more than once on the same idle task.
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210510151024.2448573-2-valentin.schneider@arm.com
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 24fd795..6a5124c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8234,12 +8234,25 @@ void __init init_idle(struct task_struct *idle, int cpu)
__sched_fork(0, idle);
+ /*
+ * The idle task doesn't need the kthread struct to function, but it
+ * is dressed up as a per-CPU kthread and thus needs to play the part
+ * if we want to avoid special-casing it in code that deals with per-CPU
+ * kthreads.
+ */
+ set_kthread_struct(idle);
+
raw_spin_lock_irqsave(&idle->pi_lock, flags);
raw_spin_rq_lock(rq);
idle->state = TASK_RUNNING;
idle->se.exec_start = sched_clock();
- idle->flags |= PF_IDLE;
+ /*
+ * PF_KTHREAD should already be set at this point; regardless, make it
+ * look like a proper per-CPU kthread.
+ */
+ idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
+ kthread_set_per_cpu(idle, cpu);
scs_task_reset(idle);
kasan_unpoison_task_stack(idle);
@@ -8456,12 +8469,8 @@ static void balance_push(struct rq *rq)
/*
* Both the cpu-hotplug and stop task are in this case and are
* required to complete the hotplug process.
- *
- * XXX: the idle task does not match kthread_is_per_cpu() due to
- * histerical raisins.
*/
- if (rq->idle == push_task ||
- kthread_is_per_cpu(push_task) ||
+ if (kthread_is_per_cpu(push_task) ||
is_migration_disabled(push_task)) {
/*
