linux-kernel - [PATCH 2/2] locking/rwbase_rt: Lockless reader waking up a writer

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Date:   Wed,  1 Sep 2021 15:28:25 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     tglx@...utronix.de
Cc:     peterz@...radead.org, mingo@...nel.org, rostedt@...dmis.org,
        longman@...hat.com, bigeasy@...utronix.de, boqun.feng@...il.com,
        dave@...olabs.net, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 2/2] locking/rwbase_rt: Lockless reader waking up a writer

As with the rest of our sleeping locks, use a wake_q to
allow waking up the writer without having to hold the
wait_lock across the operation. While this is ideally
for batching wakeups, single wakeup usage as still shown
to be beneficial vs the cost of try_to_wakeup when the
lock is contended.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 kernel/locking/rwbase_rt.c |  4 +++-
 kernel/sched/core.c        | 16 +++++++++++++---
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index 4ba15088e640..3444bc709973 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -141,6 +141,7 @@ static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 {
 	struct rt_mutex_base *rtm = &rwb->rtmutex;
 	struct task_struct *owner;
+	DEFINE_WAKE_Q(wake_q);
 
 	raw_spin_lock_irq(&rtm->wait_lock);
 	/*
@@ -151,9 +152,10 @@ static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 	 */
 	owner = rt_mutex_owner(rtm);
 	if (owner)
-		wake_up_state(owner, state);
+		wake_q_add(&wake_q, owner);
 
 	raw_spin_unlock_irq(&rtm->wait_lock);
+	wake_up_q_state(&wake_q, state);
 }
 
 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7fc3d22bc6d8..22c77742f1a7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4151,7 +4151,7 @@ void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task)
 		put_task_struct(task);
 }
 
-void wake_up_q(struct wake_q_head *head)
+static void __wake_up_q(struct wake_q_head *head, unsigned int state)
 {
 	struct wake_q_node *node = head->first;
 
@@ -4164,14 +4164,24 @@ void wake_up_q(struct wake_q_head *head)
 		task->wake_q.next = NULL;
 
 		/*
-		 * wake_up_process() executes a full barrier, which pairs with
+		 * try_to_wake_up() executes a full barrier, which pairs with
 		 * the queueing in wake_q_add() so as not to miss wakeups.
 		 */
-		wake_up_process(task);
+		try_to_wake_up(task, state, 0);
 		put_task_struct(task);
 	}
 }
 
+void wake_up_q(struct wake_q_head *head)
+{
+	__wake_up_q(head, TASK_NORMAL);
+}
+
+void wake_up_q_state(struct wake_q_head *head, unsigned int state)
+{
+	__wake_up_q(head, state);
+}
+
 /*
  * Perform scheduler related setup for a newly forked process p.
  * p is forked by current.
-- 
2.26.2