Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

[Sergey Senozhatsky]

On (06/30/17 22:38), Sergey Senozhatsky wrote:
> On (06/30/17 15:16), Petr Mladek wrote:
> > Anyway, the handshake during offloading might be pretty
> > problematic. To be honest, I do not have much experience
> > with it. I have shared some my fears in the other mail[1].
> > Jan Kara spent a lot of time on this and probably could
> > say more.
> > 
> > Maybe, we could try to look into the throotling path. Slowing down
> > massive printk() callers looks necessary when things gets
> > out of control.
> 
> throttling, in some form, is already there. I think.
> 
> there is a printk_delay() function. which we can silently activate
> when things don't look cool anymore. and printk_delay() is already
> getting called on every vprintk_emit() entry. the question is -- how
> big should be our delay value, and... when do we need to activate
> printk_delay()?
> 
> when the distance between console_seq and log_next_seq... suggests
> that we will drop (overwrite) un-flushed messages sooner than console_seq
> reaches log_next_seq? so log_next_seq is closer to log_first_seq than
> console_seq to log_next_seq.

something like below, may be. a sketch, just to demonstrate the
idea. but, once polished, can go to printk out of series.

===8<===8<===8<===

Throttle printk() callers when we detect that consoles are
far behind the logbuf: we printed to the consoles 4 times
less messages than we still have to print.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 kernel/printk/printk.c | 38 ++++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f24d3789faa0..fd546bd95207 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1758,17 +1758,43 @@ static void call_console_drivers(const char *ext_text, size_t ext_len,
 	}
 }
 
+#define PRINTK_FLOOD_DEFAULT_DELAY	10
+
 int printk_delay_msec __read_mostly;
 
+static inline void __printk_delay(int m)
+{
+	while (m--) {
+		mdelay(1);
+		touch_nmi_watchdog();
+	}
+}
+
 static inline void printk_delay(void)
 {
-	if (unlikely(printk_delay_msec)) {
-		int m = printk_delay_msec;
+	unsigned long flags;
+	u64 console_seen = 0, console_to_see;
 
-		while (m--) {
-			mdelay(1);
-			touch_nmi_watchdog();
-		}
+	if (printk_delay_msec) {
+		__printk_delay(printk_delay_msec);
+		return;
+	}
+
+	/*
+	 * Check if consoles are far behind the loguf head and
+	 * throttle printk() callers if so.
+	 */
+	logbuf_lock_irqsave(flags);
+	if (console_seq > log_first_seq)
+		console_seen = console_seq - log_first_seq;
+	console_to_see = log_next_seq - console_seq;
+	logbuf_unlock_irqrestore(flags);
+
+	if (console_seen < 4 * console_to_see) {
+		if (printk_delay_msec)
+			__printk_delay(printk_delay_msec);
+		else
+			__printk_delay(PRINTK_FLOOD_DEFAULT_DELAY);
 	}
 }
 
-- 
2.13.2