linux-kernel - [PATCH 8/8] refscale: Add SRCU-fast-updown readers

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Message-Id: <20251102224948.3906224-8-paulmck@kernel.org>
Date: Sun,  2 Nov 2025 14:49:48 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	bpf@...r.kernel.org
Subject: [PATCH 8/8] refscale: Add SRCU-fast-updown readers

This commit adds refscale readers based on srcu_read_lock_fast_updown()
and srcu_read_lock_fast_updown() ("refscale.scale_type=srcu-fast-updown").
On my x86 laptop, these are about 2.2ns per pair.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: <bpf@...r.kernel.org>
---
 kernel/rcu/refscale.c | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
index 7429ec9f0092..07a313782dfd 100644
--- a/kernel/rcu/refscale.c
+++ b/kernel/rcu/refscale.c
@@ -186,6 +186,7 @@ static const struct ref_scale_ops rcu_ops = {
 // Definitions for SRCU ref scale testing.
 DEFINE_STATIC_SRCU(srcu_refctl_scale);
 DEFINE_STATIC_SRCU_FAST(srcu_fast_refctl_scale);
+DEFINE_STATIC_SRCU_FAST_UPDOWN(srcu_fast_updown_refctl_scale);
 static struct srcu_struct *srcu_ctlp = &srcu_refctl_scale;
 
 static void srcu_ref_scale_read_section(const int nloops)
@@ -254,6 +255,42 @@ static const struct ref_scale_ops srcu_fast_ops = {
 	.name		= "srcu-fast"
 };
 
+static bool srcu_fast_updown_sync_scale_init(void)
+{
+	srcu_ctlp = &srcu_fast_updown_refctl_scale;
+	return true;
+}
+
+static void srcu_fast_updown_ref_scale_read_section(const int nloops)
+{
+	int i;
+	struct srcu_ctr __percpu *scp;
+
+	for (i = nloops; i >= 0; i--) {
+		scp = srcu_read_lock_fast_updown(srcu_ctlp);
+		srcu_read_unlock_fast_updown(srcu_ctlp, scp);
+	}
+}
+
+static void srcu_fast_updown_ref_scale_delay_section(const int nloops, const int udl, const int ndl)
+{
+	int i;
+	struct srcu_ctr __percpu *scp;
+
+	for (i = nloops; i >= 0; i--) {
+		scp = srcu_read_lock_fast_updown(srcu_ctlp);
+		un_delay(udl, ndl);
+		srcu_read_unlock_fast_updown(srcu_ctlp, scp);
+	}
+}
+
+static const struct ref_scale_ops srcu_fast_updown_ops = {
+	.init		= srcu_fast_updown_sync_scale_init,
+	.readsection	= srcu_fast_updown_ref_scale_read_section,
+	.delaysection	= srcu_fast_updown_ref_scale_delay_section,
+	.name		= "srcu-fast-updown"
+};
+
 #ifdef CONFIG_TASKS_RCU
 
 // Definitions for RCU Tasks ref scale testing: Empty read markers.
@@ -1479,7 +1516,8 @@ ref_scale_init(void)
 	long i;
 	int firsterr = 0;
 	static const struct ref_scale_ops *scale_ops[] = {
-		&rcu_ops, &srcu_ops, &srcu_fast_ops, RCU_TRACE_OPS RCU_TASKS_OPS
+		&rcu_ops, &srcu_ops, &srcu_fast_ops, &srcu_fast_updown_ops,
+		RCU_TRACE_OPS RCU_TASKS_OPS
 		&refcnt_ops, &percpuinc_ops, &incpercpu_ops, &incpercpupreempt_ops,
 		&incpercpubh_ops, &incpercpuirqsave_ops,
 		&rwlock_ops, &rwsem_ops, &lock_ops, &lock_irq_ops, &acqrel_ops,
-- 
2.40.1