[RFC][PATCHv6 08/12] printk: force printk_kthread to offload printing

Sergey Senozhatsky
2017-12-04T13:49:42Z

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@gmail.com>
---
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