[PATCH] tick/rcu: fix report_idle_softirq

[PATCH] tick/rcu: fix report_idle_softirq

[wen.yang]

From: Wen Yang <wen.yang@linux.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@linux.dev>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.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

[tip: timers/urgent] tick/sched: Fix bogus condition in report_idle_softirq()

[tip-bot2 for Wen Yang]

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     807e0d187da4c0b22036b5e34000f7a8c52f6e50
Gitweb:        https://git.kernel.org/tip/807e0d187da4c0b22036b5e34000f7a8c52f6e50
Author:        Wen Yang <wen.yang@linux.dev>
AuthorDate:    Thu, 20 Nov 2025 01:45:25 +08:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 19 Nov 2025 19:30:45 +01:00

tick/sched: Fix bogus condition in report_idle_softirq()

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
ratelimit was essentially set to zero instead of ten, and hence *no*
softirq pending messages would ever be issued, but "fixed" it as:

-       if (ratelimit < 10)
+       if (ratelimit >= 10)
                return false;

However, this fix introduced another issue:

When ratelimit is greater than or equal 10, even if A is true, it will
directly return false. While ratelimit in the original code was only used
to control printing and will not affect the return value.

Restore the original logic and restrict ratelimit to control the printk and
not the return value.

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@linux.dev>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251119174525.29470-1-wen.yang@linux.dev
---
 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 c527b42..466e083 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1152,16 +1152,15 @@ 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;
 }