linux-kernel - [PATCH] tick/rcu: fix report_idle

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Message-Id: <20251119174525.29470-1-wen.yang@linux.dev>
Date: Thu, 20 Nov 2025 01:45:25 +0800
From: wen.yang@...ux.dev
To: Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc: Wen Yang <wen.yang@...ux.dev>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] tick/rcu: fix report_idle_softirq

From: Wen Yang <wen.yang@...ux.dev>

In commit 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle") the
new function report_idle_softirq() was created by breaking code out of the
existing can_stop_idle_tick() for kernels v5.18 and newer.

In doing so, the code essentially went from this form:

	if (A) {
		static int ratelimit;
		if (ratelimit < 10 && !C && A&D) {
                       pr_warn("NOHZ tick-stop error: ...");
		       ratelimit++;
		}
		return false;
	}

to a new function:

static bool report_idle_softirq(void)
{
       static int ratelimit;

       if (likely(!A))
               return false;

       if (ratelimit < 10)
               return false;
...
       pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
               pending);
       ratelimit++;

       return true;
}

commit a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition") realized
the ratelimit was essentially set to zero instead of ten, and hence *no*
softirq pending messages would ever be issued, but fix it as:
-       if (ratelimit < 10)
+       if (ratelimit >= 10)
                return false;

However, this fix introduces another issue:
When ratelimit is greater than 10, even if A is true, it will directly
return false. While ratelimit in the original code is only used to control
printing and will not affect the return value.
This patch fixes the above issues.

Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
Fixes: a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition")
Signed-off-by: Wen Yang <wen.yang@...ux.dev>
Cc: Paul Gortmaker <paul.gortmaker@...driver.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Frederic Weisbecker <frederic@...nel.org>
Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org
---
 kernel/time/tick-sched.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3ff3eb1f90d0..8ddf74e705d3 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1174,16 +1174,15 @@ static bool report_idle_softirq(void)
 			return false;
 	}
 
-	if (ratelimit >= 10)
-		return false;
-
 	/* On RT, softirq handling may be waiting on some lock */
 	if (local_bh_blocked())
 		return false;
 
-	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
-		pending);
-	ratelimit++;
+	if (ratelimit < 10) {
+		pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
+			pending);
+		ratelimit++;
+	}
 
 	return true;
 }
-- 
2.25.1