From 20b641ecb0f0724979a89433b636899707661c35 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Tue, 19 Oct 2021 02:08:14 +0200
Subject: [PATCH] rcu: Fix callbacks processing time limit retaining
 cond_resched()

References: bnc#1189998 (PREEMPT_RT prerequisite backports)
Patch-mainline: v5.17-rc0
Git-commit: 3e61e95e2d095e308616cba4ffb640f95a480e01

The callbacks processing time limit makes sure we are not exceeding a
given amount of time executing the queue.

However its "continue" clause bypasses the cond_resched() call on
rcuc and NOCB kthreads, delaying it until we reach the limit, which can
be very long...

Make sure the scheduler has a higher priority than the time limit.

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/rcu/tree.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0476adc1137a..688d6da34c50 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2534,9 +2534,21 @@ static void rcu_do_batch(struct rcu_data *rdp)
 		/*
 		 * Stop only if limit reached and CPU has something to do.
 		 */
-		if (count >= bl && in_serving_softirq() &&
-		    (need_resched() || !is_idle_task(current)))
-			break;
+		if (in_serving_softirq()) {
+			if (count >= bl && (need_resched() || !is_idle_task(current)))
+				break;
+		} else {
+			local_bh_enable();
+			lockdep_assert_irqs_enabled();
+			cond_resched_tasks_rcu_qs();
+			lockdep_assert_irqs_enabled();
+			local_bh_disable();
+		}
+
+		/*
+		 * Make sure we don't spend too much time here and deprive other
+		 * softirq vectors of CPU cycles.
+		 */
 		if (unlikely(tlimit)) {
 			/* only call local_clock() every 32 callbacks */
 			if (likely((count & 31) || local_clock() < tlimit))
@@ -2544,13 +2556,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
 			/* Exceeded the time limit, so leave. */
 			break;
 		}
-		if (!in_serving_softirq()) {
-			local_bh_enable();
-			lockdep_assert_irqs_enabled();
-			cond_resched_tasks_rcu_qs();
-			lockdep_assert_irqs_enabled();
-			local_bh_disable();
-		}
 	}
 
 	rcu_nocb_lock_irqsave(rdp, flags);