[PATCHv2 2/2] kernel/kexec: Stop all userspace deadline tasks

[Pingfan Liu <piliu@redhat.com>]

Now that the deadline bandwidth check has been skipped, there is nothing
to prevent CPUs from being unplugged. But as deadline tasks are crowded
onto the remaining CPUs, they may starve normal tasks, especially the
hotplug kthreads. As a result, the kexec process will hang indefinitely.

Send SIGSTOP to all userspace deadline tasks at the beginning of kexec
to allow other tasks to run as CPUs are unplugged.

Signed-off-by: Pingfan Liu <piliu@redhat.com>
---
 kernel/kexec_core.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 265de9d1ff5f5..090bb58797d12 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1132,6 +1132,18 @@ bool kexec_load_permitted(int kexec_image_type)
 	return true;
 }
 
+static void stop_user_deadline_tasks(void)
+{
+	struct task_struct *task;
+
+	rcu_read_lock();
+	for_each_process(task) {
+		if (task->policy == SCHED_DEADLINE && task->mm)
+			send_sig(SIGSTOP, task, 1);
+	}
+	rcu_read_unlock();
+}
+
 /*
  * Move into place and start executing a preloaded standalone
  * executable.  If nothing was preloaded return an error.
@@ -1190,6 +1202,11 @@ int kernel_kexec(void)
 		cpu_hotplug_disable();
 		kexec_in_progress = true;
 		cpu_hotplug_enable();
+		/*
+		 * As CPU hot-removal, the crowed deadline task may starve other
+		 * tasks. So stop them.
+		 */
+		stop_user_deadline_tasks();
 		kernel_restart_prepare("kexec reboot");
 		migrate_to_reboot_cpu();
 		syscore_shutdown();
-- 
2.49.0