linux-kernel - [PATCH RFC 6/9] ratelimit: Count misses due to lock contention

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Message-Id: <20250403211514.985900-6-paulmck@kernel.org>
Date: Thu,  3 Apr 2025 14:15:11 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: kernel-team@...a.com,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kuniyuki Iwashima <kuniyu@...zon.com>,
	Mateusz Guzik <mjguzik@...il.com>,
	Petr Mladek <pmladek@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [PATCH RFC 6/9] ratelimit: Count misses due to lock contention

The ___ratelimit() function simply returns zero ("do ratelimiting")
if the trylock fails, but does not adjust the ->missed field.  This
means that the resulting dropped printk()s are dropped silently, which
could seriously confuse people trying to do console-log-based debugging.
Therefore, increment the ->missed field upon trylock failure.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: Mateusz Guzik <mjguzik@...il.com>
Cc: Petr Mladek <pmladek@...e.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
---
 lib/ratelimit.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/ratelimit.c b/lib/ratelimit.c
index 18703f92d73e7..19ad3cdbd1711 100644
--- a/lib/ratelimit.c
+++ b/lib/ratelimit.c
@@ -44,8 +44,10 @@ int ___ratelimit(struct ratelimit_state *rs, const char *func)
 	 * in addition to the one that will be printed by
 	 * the entity that is holding the lock already:
 	 */
-	if (!raw_spin_trylock_irqsave(&rs->lock, flags))
+	if (!raw_spin_trylock_irqsave(&rs->lock, flags)) {
+		ratelimit_state_inc_miss(rs);
 		return 0;
+	}
 
 	if (!rs->begin)
 		rs->begin = jiffies;
-- 
2.40.1