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Subject[PATCH] kthread: run kthreadd with max priority SCHED_FIFO
kthreadd, the creator of other kernel threads, runs as a normal
priority task. This is a potential for priority inversion when a task
wants to spawn a high-priority kernel thread. A middle priority
SCHED_FIFO task can block kthreadd's execution indefinitely and thus
prevent the timely creation of the high-priority kernel thread.

This causes a practical problem. When a runaway real-time task is
eating 100% CPU and we attempt to put the CPU offline, sometimes we
block while waiting for the creation of the highest-priority
"kstopmachine" thread.

The fix is to run kthreadd with the highest possible SCHED_FIFO
priority. Its children must still run as slightly negatively reniced
SCHED_NORMAL tasks.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>

diff --git a/kernel/kthread.c b/kernel/kthread.c
index dcfe724..a7ce932 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -94,10 +94,17 @@ static void create_kthread(struct kthread_create_info *create)
if (pid < 0) {
create->result = ERR_PTR(pid);
} else {
+ struct sched_param param = { .sched_priority = 0 };
wait_for_completion(&create->started);
read_lock(&tasklist_lock);
create->result = find_task_by_pid(pid);
read_unlock(&tasklist_lock);
+ /*
+ * We (kthreadd) run with SCHED_FIFO, but we don't want
+ * the kthreads we create to have it too by default.
+ */
+ sched_setscheduler(create->result, SCHED_NORMAL, &param);
+ set_user_nice(create->result, -5);
}
complete(&create->done);
}
@@ -217,11 +224,12 @@ EXPORT_SYMBOL(kthread_stop);
int kthreadd(void *unused)
{
struct task_struct *tsk = current;
+ struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };

/* Setup a clean context for our children to inherit. */
set_task_comm(tsk, "kthreadd");
ignore_signals(tsk);
- set_user_nice(tsk, -5);
+ sched_setscheduler(tsk, SCHED_FIFO, &param);
set_cpus_allowed(tsk, CPU_MASK_ALL);

current->flags |= PF_NOFREEZE;

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