[RFC][PATCH 2/3] printk: lock/unlock console only for new logbuf entries

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

From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>

Prior to 5c2992ee7fd8a29 ("printk: remove console flushing special
cases for partial buffered lines") we would do console_cont_flush()
for each pr_cont() to print cont fragments, so console_unlock() would
actually print data:

	pr_cont();
	 console_lock();
	 console_unlock()
	  console_cont_flush(); // print cont fragment
	...
	pr_cont();
	 console_lock();
	 console_unlock()
	  console_cont_flush(); // print cont fragment

We don't do console_cont_flush() anymore, so when we do pr_cont()
console_unlock() does nothing (unless we flushed the cont buffer):

	pr_cont();
	 console_lock();
	 console_unlock();      // noop
	...
	pr_cont();
	 console_lock();
	 console_unlock();      // noop
	...
	pr_cont();
	  cont_flush();
	    console_lock();
	    console_unlock();   // print data

We also wakeup klogd purposelessly for pr_cont() output - un-flushed
cont buffer is not stored in log_buf; there is nothing to pull.

Thus we can console_lock()/console_unlock()/wake_up_klogd() only when
we know that we log_store()-ed a message and there is something to
print to the consoles/syslog.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/printk.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e72cb793aff1..aea37b7927dd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1916,8 +1916,9 @@ asmlinkage int vprintk_emit(int facility, int level,
 			    const char *fmt, va_list args)
 {
 	int printed_len;
-	bool in_sched = false;
+	bool in_sched = false, pending_output;
 	unsigned long flags;
+	u64 curr_log_seq;
 
 	if (level == LOGLEVEL_SCHED) {
 		level = LOGLEVEL_DEFAULT;
@@ -1929,11 +1930,13 @@ asmlinkage int vprintk_emit(int facility, int level,
 
 	/* This stops the holder of console_sem just where we want him */
 	logbuf_lock_irqsave(flags);
+	curr_log_seq = log_next_seq;
 	printed_len = vprintk_store(facility, level, dict, dictlen, fmt, args);
+	pending_output = (curr_log_seq != log_next_seq);
 	logbuf_unlock_irqrestore(flags);
 
 	/* If called from the scheduler, we can not call up(). */
-	if (!in_sched) {
+	if (!in_sched && pending_output) {
 		/*
 		 * Disable preemption to avoid being preempted while holding
 		 * console_sem which would prevent anyone from printing to
@@ -1950,7 +1953,8 @@ asmlinkage int vprintk_emit(int facility, int level,
 		preempt_enable();
 	}
 
-	wake_up_klogd();
+	if (pending_output)
+		wake_up_klogd();
 	return printed_len;
 }
 EXPORT_SYMBOL(vprintk_emit);
-- 
2.19.0