linux-kernel - [PATCH 2/2] tick/nohz: WARN_ON --> WARN_ON

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Date:   Mon, 21 Feb 2022 13:20:09 -0500
From:   Paul Gortmaker <paul.gortmaker@...driver.com>
To:     <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 2/2] tick/nohz: WARN_ON --> WARN_ON_ONCE to prevent console saturation

While running some testing on code that happened to allow the variable
tick_nohz_full_running to get set but with no "possible" NOHZ cores to
back up that setting, I tripped this WARN:

        if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
                WARN_ON(tick_nohz_full_running);

The console was overwhemled with an endless stream of one WARN per tick
per core and there was no way to even see what was going on w/o using a
serial console to capture it and then trace it back to this guy.

Changing it to ONCE reveals that we get the message we need in a
civilized fashion, and the system can limp along until rebooted.

Fixes: 08ae95f4fd3b ("nohz_full: Allow the boot CPU to be nohz_full")
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 kernel/time/tick-sched.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 17a283ce2b20..5e80ee44c32a 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -186,7 +186,7 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 	 */
 	if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)) {
 #ifdef CONFIG_NO_HZ_FULL
-		WARN_ON(tick_nohz_full_running);
+		WARN_ON_ONCE(tick_nohz_full_running);
 #endif
 		tick_do_timer_cpu = cpu;
 	}
-- 
2.17.1