linux-kernel - [PATCH 5.15 104/135] rcu: Fix callbacks processing time limit retaining cond

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Message-Id: <20220510130743.390330035@linuxfoundation.org>
Date:   Tue, 10 May 2022 15:08:06 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH 5.15 104/135] rcu: Fix callbacks processing time limit retaining cond_resched()

From: Frederic Weisbecker <frederic@...nel.org>

commit 3e61e95e2d095e308616cba4ffb640f95a480e01 upstream.

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@....com>
Tested-by: Valentin Schneider <valentin.schneider@....com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Valentin Schneider <valentin.schneider@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
[UR: backport to 5.15-stable + commit update]
Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/rcu/tree.c |   27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2513,10 +2513,22 @@ static void rcu_do_batch(struct rcu_data
 		/*
 		 * Stop only if limit reached and CPU has something to do.
 		 */
-		if (count >= bl && !offloaded &&
-		    (need_resched() ||
-		     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
-			break;
+		if (in_serving_softirq()) {
+			if (count >= bl && (need_resched() ||
+					(!is_idle_task(current) && !rcu_is_callbacks_kthread())))
+				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))
@@ -2524,13 +2536,6 @@ static void rcu_do_batch(struct rcu_data
 			/* 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();
-		}
 	}
 
 	local_irq_save(flags);