linux-kernel - [PATCH rcu 1/3] srcu: Expedite SRCU-fast grace periods

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Message-Id: <20250709104555.15670-2-neeraj.upadhyay@kernel.org>
Date: Wed,  9 Jul 2025 16:15:53 +0530
From: neeraj.upadhyay@...nel.org
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	paulmck@...nel.org,
	joelagnelf@...dia.com,
	frederic@...nel.org,
	boqun.feng@...il.com,
	urezki@...il.com,
	rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com,
	jiangshanlai@...il.com,
	qiang.zhang1211@...il.com,
	neeraj.iitr10@...il.com,
	neeraj.upadhyay@....com,
	"Neeraj Upadhyay (AMD)" <neeraj.upadhyay@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>
Subject: [PATCH rcu 1/3] srcu: Expedite SRCU-fast grace periods

From: "Paul E. McKenney" <paulmck@...nel.org>

Currently, SRCU-fast grace periods use synchronize_rcu() to provide the
needed ordering with readers, even given an expedited SRCU-fast grace
period, which isn't all that expedited.  This commit therefore instead
uses synchronize_rcu_expedited() if there is an expedited SRCU-fast
grace period in flight.

Of course, given an non-expedited SRCU-fast grace period blocked in
synchronize_rcu(), a later request for an expedited SRCU-fast grace
period will wait for that synchronize_rcu() to return before switching
to use of synchronize_rcu_expedited().  If this turns out to be a real
problem for a production workload, we can increase the complexity (but
likely also degrade the energy efficiency) to speed things up further.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@...nel.org>
---
 kernel/rcu/srcutree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 48047260697e..c5e8ebc493d5 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -502,6 +502,8 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
 	 */
 	if (!did_gp)
 		smp_mb(); /* A */
+	else if (srcu_gp_is_expedited(ssp))
+		synchronize_rcu_expedited(); /* X */
 	else
 		synchronize_rcu(); /* X */

-- 
2.40.1