[RFC][PATCHv5 10/13] printk: force printk_kthread to offload printing

[Sergey Senozhatsky]

As of now we don't `offload' printing from printk_kthread and
print all pending logbuf messages. This, however, may have a
negative effect. We still hold console_sem as long as we have
got messages to print, and there might be other console_lock()
callers sleeping on console_sem in TASK_UNINTERRUPTIBLE,
including user space processes (tty_open, drm IOCTL, etc.).
So we need to up() console_sem every once in a while, even if
current console_sem owner is printk_kthread, just in order to
wake up those other processes that can sleep on console_sem.

If there are no tasks skeeping on console_sem, then printk_kthread
will immediately return back to console_unclok(), because we don't
clear the PRINTK_PENDING_OUTPUT bit and printk_kthread checks it
before it decides to schedule().

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 kernel/printk/printk.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 71950bd85eac..6da4e21c3b45 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -600,15 +600,29 @@ static inline bool console_offload_printing(void)
 		return false;
 	}
 
-	/* Once we offloaded to printk_ktread - keep printing */
-	if (current == printk_kthread)
-		return false;
-
 	adj_atomic_print_limit();
 
 	if (!time_after_eq(now, printing_start_ts + atomic_print_limit))
 		return false;
 
+	if (current == printk_kthread) {
+		/*
+		 * All tasks are equal, all tasks must offload. However,
+		 * printk_kthread may be the only process left willing to
+		 * down(). So we may return here immediately after we leave,
+		 * because of positive PRINTK_PENDING_OUTPUT check in
+		 * printk_kthread_func() loop. However, we obviously don't
+		 * want to declare printk emergency in this case, so that's
+		 * why we update `printing_start_ts' here.
+		 *
+		 * In case if `printk_kthread' will immediately return
+		 * back to console_unlock(), it will have another full
+		 * `atomic_print_limit' time slice.
+		 */
+		printing_start_ts = local_clock() >> 30LL;
+		return true;
+	}
+
 	/*
 	 * A trivial emergency enforcement - give up on printk_kthread if
 	 * we can't wake it up.
-- 
2.14.1