[RFC][PATCHv6 05/12] printk: enable printk offloading

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Initialize kernel printing thread and make printk offloading
possible. By default `offloading_enabled' module parameter is
set to `false', so no offloading will take place, unless
requested by user.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/printk.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 16f5f5c7e541..a427cee2aa00 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2914,6 +2914,65 @@ static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
 	.flags = IRQ_WORK_LAZY,
 };
 
+static int printk_kthread_func(void *data)
+{
+	while (1) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		/*
+		 * We must check `printk_emergency' as well, to let
+		 * printk_emergency_begin() stop active `printk_kthread' at
+		 * some point. Otherwise we can end up in a loop:
+		 *   - we bail out of console_unlock() because of
+		 *     printk_kthread_should_stop()
+		 * and
+		 *   - don't schedule() and attempt to return back
+		 *     immediately to console_unlock() because we
+		 *     see PRINTK_PENDING_OUTPUT bit set.
+		 */
+		if (!test_bit(PRINTK_PENDING_OUTPUT, &printk_pending) ||
+				atomic_read(&printk_emergency) != 0)
+			schedule();
+
+		__set_current_state(TASK_RUNNING);
+
+		/* We might have been woken for stop */
+		if (kthread_should_park())
+			kthread_parkme();
+
+		console_lock();
+		console_unlock();
+
+		/* We might have been blocked on console_sem */
+		if (kthread_should_park())
+			kthread_parkme();
+	}
+
+	return 0;
+}
+
+/*
+ * Init printk kthread at late_initcall stage, after core/arch/device/etc.
+ * initialization.
+ */
+static int __init init_printk_kthread(void)
+{
+	struct task_struct *thread;
+
+	if (!alloc_cpumask_var(&printk_offload_cpus, GFP_KERNEL))
+		return -ENOMEM;
+
+	thread = kthread_run(printk_kthread_func, NULL, "printk");
+	if (IS_ERR(thread)) {
+		pr_err("printk: unable to create printing thread\n");
+		free_cpumask_var(printk_offload_cpus);
+		return PTR_ERR(thread);
+	}
+
+	printk_kthread = thread;
+	return 0;
+}
+late_initcall(init_printk_kthread);
+
 void wake_up_klogd(void)
 {
 	preempt_disable();
-- 
2.15.1