[PATCH 2/3] rcu: Enable rcu_normal_wake_from_gp on small systems

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Automatically enable the rcu_normal_wake_from_gp parameter on
systems with a small number of CPUs. The activation threshold
is set to 16 CPUs.

This helps to reduce a latency of normal synchronize_rcu() API
by waking up GP-waiters earlier and decoupling synchronize_rcu()
callers from regular callback handling.

A benchmark running 64 parallel jobs invoking synchronize_rcu()
demonstrates a notable latency reduction with the setting enabled.

Latency distribution (microseconds):

<default>
 0      - 9999   : 1
 10000  - 19999  : 4
 20000  - 29999  : 399
 30000  - 39999  : 3197
 40000  - 49999  : 10428
 50000  - 59999  : 17363
 60000  - 69999  : 15529
 70000  - 79999  : 9287
 80000  - 89999  : 4249
 90000  - 99999  : 1915
 100000 - 109999 : 922
 110000 - 119999 : 390
 120000 - 129999 : 187
 ...
<default>

<rcu_normal_wake_from_gp>
 0      - 9999  : 1
 10000  - 19999 : 234
 20000  - 29999 : 6678
 30000  - 39999 : 33463
 40000  - 49999 : 20669
 50000  - 59999 : 2766
 60000  - 69999 : 183
 ...
<rcu_normal_wake_from_gp>

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 14d4499c6fc3..c0e0b38a08dc 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1625,7 +1625,9 @@ static void rcu_sr_put_wait_head(struct llist_node *node)
 	atomic_set_release(&sr_wn->inuse, 0);
 }
 
-/* Disabled by default. */
+/* Enable rcu_normal_wake_from_gp automatically on small systems. */
+#define WAKE_FROM_GP_CPU_THRESHOLD 16
+
 static int rcu_normal_wake_from_gp;
 module_param(rcu_normal_wake_from_gp, int, 0644);
 static struct workqueue_struct *sync_wq;
@@ -4847,6 +4849,9 @@ void __init rcu_init(void)
 	sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0);
 	WARN_ON(!sync_wq);
 
+	if (num_possible_cpus() <= WAKE_FROM_GP_CPU_THRESHOLD)
+		WRITE_ONCE(rcu_normal_wake_from_gp, 1);
+
 	/* Fill in default value for rcutree.qovld boot parameter. */
 	/* -After- the rcu_node ->lock fields are initialized! */
 	if (qovld < 0)
-- 
2.39.5