[PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker

Frederic Weisbecker
2023-03-22T19:45:47Z

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 cb57e8312231..a3dc7465b0b2 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1350,12 +1350,20 @@ 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