Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E . McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        rcu <rcu@vger.kernel.org>, Uladzislau Rezki <urezki@gmail.com>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>
Subject: [PATCH 4/4] rcu/nocb: Make shrinker to iterate only NOCB CPUs
Date:   Wed, 22 Mar 2023 20:44:56 +0100
Message-Id: <20230322194456.2331527-5-frederic@kernel.org>
In-Reply-To: <20230322194456.2331527-1-frederic@kernel.org>
References: <20230322194456.2331527-1-frederic@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Callbacks can only be queued as lazy on NOCB CPUs, therefore iterating
over the NOCB mask is enough for both counting and scanning. Just lock
the mostly uncontended barrier mutex on counting as well in order to
keep rcu_nocb_mask stable.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree_nocb.h | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index a3dc7465b0b2..185c0c9a60d4 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1319,13 +1319,21 @@ lazy_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 	int cpu;
 	unsigned long count = 0;
 
+	if (WARN_ON_ONCE(!cpumask_available(rcu_nocb_mask)))
+		return 0;
+
+	/*  Protect rcu_nocb_mask against concurrent (de-)offloading. */
+	mutex_lock(&rcu_state.barrier_mutex);
+
 	/* Snapshot count of all CPUs */
-	for_each_possible_cpu(cpu) {
+	for_each_cpu(cpu, rcu_nocb_mask) {
 		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 
 		count +=  READ_ONCE(rdp->lazy_len);
 	}
 
+	mutex_unlock(&rcu_state.barrier_mutex);
+
 	return count ? count : SHRINK_EMPTY;
 }
 
@@ -1336,6 +1344,8 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 	unsigned long flags;
 	unsigned long count = 0;
 
+	if (WARN_ON_ONCE(!cpumask_available(rcu_nocb_mask)))
+		return 0;
 	/*
 	 * Protect against concurrent (de-)offloading. Otherwise nocb locking
 	 * may be ignored or imbalanced.
@@ -1343,7 +1353,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 	mutex_lock(&rcu_state.barrier_mutex);
 
 	/* Snapshot count of all CPUs */
-	for_each_possible_cpu(cpu) {
+	for_each_cpu(cpu, rcu_nocb_mask) {
 		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 		int _count;
 
-- 
2.34.1