[RFC][PATCHv6 08/12] 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.).

A bigger issue is that we still can schedule with console_sem
locked, which is not right and we want to avoid it. This patch
also serves as a preparation for preemption disabled printing
loop.

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 sleeping 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 | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 372c71f69e8e..4d91182e25e3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -586,14 +586,27 @@ static inline bool should_handoff_printing(u64 printing_start_ts)
 	if (time_after_eq64(now, printing_start_ts))
 		printing_elapsed += now - printing_start_ts;
 
-	/* Once we offloaded to printk_ktread - keep printing */
-	if (current == printk_kthread)
-		return false;
-
 	/* Shrink down to seconds and check the offloading threshold */
 	if ((printing_elapsed >> 30LL) < offloading_threshold())
 		return false;
 
+	if (current == printk_kthread) {
+		/*
+		 * All tasks must offload - we don't want to keep console_sem
+		 * locked for too long. However, printk_kthread may be the
+		 * only process left willing to down(). So we may return back
+		 * immediately after we leave, because PRINTK_PENDING_OUTPUT
+		 * bit is still set and printk_kthread_func() won't schedule.
+		 * This still counts as offloading, so reset the stats.
+		 *
+		 * Should `printk_kthread' immediately return back to
+		 * console_unlock(), it will have another full
+		 * `offloading_threshold()' time slice.
+		 */
+		printing_elapsed = 0;
+		return true;
+	}
+
 	/*
 	 * We try to set `printk_kthread' CPU affinity to any online CPU
 	 * except for this_cpu. Because otherwise `printk_kthread' may be
-- 
2.15.1