[RFC][PATCHv3 4/5] printk: enable printk offloading

[Sergey Senozhatsky]

Initialize kernel printing thread and make printk offloading
possible. By default `atomic_print_limit' is set to 0, so no
offloading will take place, unless requested by user.

Signed-off-by: Sergey Senozhatsky <sergey.senozhat...@gmail.com>
---
 kernel/printk/printk.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ba640fbcee7c..81ea575728b9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -49,6 +49,7 @@
 #include <linux/sched/debug.h>
 #include <linux/sched/task_stack.h>
 #include <linux/kthread.h>
+#include <uapi/linux/sched/types.h>
 
 #include <linux/uaccess.h>
 #include <asm/sections.h>
@@ -2927,6 +2928,43 @@ 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);
+               if (!test_bit(PRINTK_PENDING_OUTPUT, &printk_pending))
+                       schedule();
+
+               __set_current_state(TASK_RUNNING);
+
+               console_lock();
+               console_unlock();
+       }
+
+       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;
+       struct sched_param param = { .sched_priority = MAX_USER_RT_PRIO / 2 };
+
+       thread = kthread_run(printk_kthread_func, NULL, "printk");
+       if (IS_ERR(thread)) {
+               pr_err("printk: unable to create printing thread\n");
+               return PTR_ERR(thread);
+       }
+
+       sched_setscheduler(thread, SCHED_FIFO, &param);
+       printk_kthread = thread;
+       return 0;
+}
+late_initcall(init_printk_kthread);
+
 void wake_up_klogd(void)
 {
        preempt_disable();
-- 
2.12.2