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 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker
Date: Wed, 29 Mar 2023 18:02:02 +0200 [thread overview]
Message-ID: <20230329160203.191380-4-frederic@kernel.org> (raw)
In-Reply-To: <20230329160203.191380-1-frederic@kernel.org>
The ->lazy_len is only checked locklessly. Recheck again under the
->nocb_lock to avoid spending more time on flushing/waking if not
necessary. The ->lazy_len can still increment concurrently (from 1 to
infinity) but under the ->nocb_lock we at least know for sure if there
are lazy callbacks at all (->lazy_len > 0).
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree_nocb.h | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index c321fce2af8e..dfa9c10d6727 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1358,12 +1358,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
if (!rcu_rdp_is_offloaded(rdp))
continue;
+ if (!READ_ONCE(rdp->lazy_len))
+ continue;
+
+ rcu_nocb_lock_irqsave(rdp, flags);
+ /*
+ * Recheck under the nocb lock. Since we are not holding the bypass
+ * lock we may still race with increments from the enqueuer but still
+ * we know for sure if there is at least one lazy callback.
+ */
_count = READ_ONCE(rdp->lazy_len);
-
- if (_count == 0)
+ if (!_count) {
+ rcu_nocb_unlock_irqrestore(rdp, flags);
continue;
-
- rcu_nocb_lock_irqsave(rdp, flags);
+ }
WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
rcu_nocb_unlock_irqrestore(rdp, flags);
wake_nocb_gp(rdp, false);
--
2.34.1
next prev parent reply other threads:[~2023-03-29 16:05 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-29 16:01 [PATCH 0/4 v2] rcu/nocb: Shrinker related boring fixes Frederic Weisbecker
2023-03-29 16:02 ` [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading Frederic Weisbecker
2023-03-29 20:44 ` Paul E. McKenney
2023-03-29 21:18 ` Frederic Weisbecker
2023-03-29 16:02 ` [PATCH 2/4] rcu/nocb: Fix shrinker race against callback enqueuer Frederic Weisbecker
2023-03-29 20:47 ` Paul E. McKenney
2023-03-29 16:02 ` Frederic Weisbecker [this message]
2023-03-29 20:54 ` [PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker Paul E. McKenney
2023-03-29 21:22 ` Frederic Weisbecker
2023-03-29 21:38 ` Paul E. McKenney
2023-03-29 16:02 ` [PATCH 4/4] rcu/nocb: Make shrinker to iterate only NOCB CPUs Frederic Weisbecker
2023-03-29 20:58 ` Paul E. McKenney
2023-03-29 21:35 ` Frederic Weisbecker
2023-03-29 23:12 ` Paul E. McKenney
2023-04-24 17:35 ` [PATCH 0/4 v2] rcu/nocb: Shrinker related boring fixes Paul E. McKenney
-- strict thread matches above, loose matches on Subject: below --
2023-03-22 19:44 [PATCH 0/4] " Frederic Weisbecker
2023-03-22 19:44 ` [PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker Frederic Weisbecker
2023-03-22 23:21 ` Paul E. McKenney
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