linux-kernel - [PATCH 5/9] rcu: Add rcutree.lazy

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Date:   Wed, 31 May 2023 12:17:32 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        rcu <rcu@...r.kernel.org>, Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Giovanni Gherdovich <ggherdovich@...e.cz>
Subject: [PATCH 5/9] rcu: Add rcutree.lazy_enabled boot parameter

Allow to overwrite the arbitrary default number of lazy callbacks
threshold that is currently set to 100. This allows for tuning between
powersaving, throughtput and memory consumption expectations. As a
bonus, setting this value to 0 disables lazy callbacks.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 5 +++++
 kernel/rcu/tree.c                               | 3 ++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 505978cfb548..dd2be4249061 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4798,6 +4798,11 @@
 			Set threshold of queued RCU callbacks beyond which
 			batch limiting is disabled.
 
+	rcutree.qhimark_lazy = [KNL]
+			Set threshold of queued lazy RCU callbacks beyond which
+			batch must be flushed to the main queue. If set to 0,
+			disable lazy queue.
+
 	rcutree.qlowmark= [KNL]
 			Set threshold of queued RCU callbacks below which
 			batch limiting is re-enabled.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9b98d87fa22e..e33c0d889216 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -390,6 +390,7 @@ static long qovld_calc = -1;	  // No pre-initialization lock acquisitions!
 
 module_param(blimit, long, 0444);
 module_param(qhimark, long, 0444);
+module_param(qhimark_lazy, long, 0444);
 module_param(qlowmark, long, 0444);
 module_param(qovld, long, 0444);
 
@@ -2655,7 +2656,7 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
 	kasan_record_aux_stack_noalloc(head);
 	local_irq_save(flags);
 	rdp = this_cpu_ptr(&rcu_data);
-	lazy = lazy_in && !rcu_async_should_hurry();
+	lazy = lazy_in && qhimark_lazy && !rcu_async_should_hurry();
 
 	/* Add the callback to our list. */
 	if (unlikely(!rcu_segcblist_is_enabled(&rdp->cblist))) {
-- 
2.40.1