linux-kernel - [PATCH] tick/rcu: fix false positive "softirq work is pending" messages on RT

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Message-ID: <20230818200757.1808398-1-paul.gortmaker@windriver.com>
Date:   Fri, 18 Aug 2023 16:07:57 -0400
From:   <paul.gortmaker@...driver.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>
CC:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        Wen Yang <wenyang.linux@...mail.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH] tick/rcu: fix false positive "softirq work is pending" messages on RT

From: Paul Gortmaker <paul.gortmaker@...driver.com>

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 a one conditional:

	if (a && b && c)
		warn();

to a three conditional:

	if (!a)
		return;
	if (!b)
		return;
	if (!c)
		return;
	warn();

However, it seems one of the conditionals didn't get a "!" removed.
Compare the instance of local_bh_blocked() in the old code:

-               if (ratelimit < 10 && !local_bh_blocked() &&
-                   (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
-                       pr_warn("NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #%02x!!!\n",
-                               (unsigned int) local_softirq_pending());
-                       ratelimit++;
-               }

...to the usage in the new (5.18+) code:

+       /* On RT, softirqs handling may be waiting on some lock */
+       if (!local_bh_blocked())
+               return false;

It seems apparent that the "!" should be removed from the new code.

This issue lay dormant until another fixup for the same commit was added
in commit a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition").
This commit realized the ratelimit was essentially set to zero instead
of ten, and hence *no* softirq pending messages would ever be issued.

Once this commit was backported via linux-stable, both the v6.1 and v6.4
preempt-rt kernels started printing out 10 instances of this at boot:

  NOHZ tick-stop error: local softirq work is pending, handler #80!!!

Just to double check my understanding of things, I confirmed that the
v5.18-rt did print the pending-80 messages with a cherry pick of the
ratelimit fix, and then confirmed no pending softirq messages were
printed with a revert of mainline's 034569 on a v5.18-rt baseline.

Finally I confirmed it fixed the issue on v6.1-rt and v6.4-rt, and
also didn't break anything on a defconfig of mainline master of today.

Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
Cc: Wen Yang <wenyang.linux@...mail.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2b865cb77feb..b52e1861b913 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1050,7 +1050,7 @@ static bool report_idle_softirq(void)
 		return false;

 	/* On RT, softirqs handling may be waiting on some lock */
-	if (!local_bh_blocked())
+	if (local_bh_blocked())
 		return false;

 	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
-- 
2.40.0