[PATCH v5 08/18] locking/rwsem: Always release wait_lock before waking up tasks

From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	Davidlohr Bueso <dave@stgolabs.net>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	huang ying <huang.ying.caritas@gmail.com>,
	Waiman Long <longman@redhat.com>
Subject: [PATCH v5 08/18] locking/rwsem: Always release wait_lock before waking up tasks
Date: Thu, 18 Apr 2019 19:46:18 -0400	[thread overview]
Message-ID: <20190418234628.3675-9-longman@redhat.com> (raw)
In-Reply-To: <20190418234628.3675-1-longman@redhat.com>

With the use of wake_q, we can do task wakeups without holding the
wait_lock. There is one exception in the rwsem code, though. It is
when the writer in the slowpath detects that there are waiters ahead
but the rwsem is not held by a writer. This can lead to a long wait_lock
hold time especially when a large number of readers are to be woken up.

Remediate this situation by releasing the wait_lock before waking up
tasks and re-acquiring it afterward.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/sched/wake_q.h |  5 +++++
 kernel/locking/rwsem.c       | 27 ++++++++++++++++-----------
 2 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
index ad826d2a4557..26a2013ac39c 100644
--- a/include/linux/sched/wake_q.h
+++ b/include/linux/sched/wake_q.h
@@ -51,6 +51,11 @@ static inline void wake_q_init(struct wake_q_head *head)
 	head->lastp = &head->first;
 }
 
+static inline bool wake_q_empty(struct wake_q_head *head)
+{
+	return head->first == WAKE_Q_TAIL;
+}
+
 extern void wake_q_add(struct wake_q_head *head, struct task_struct *task);
 extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task);
 extern void wake_up_q(struct wake_q_head *head);
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 82cfc5a1c42d..087bbef2089e 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -731,17 +731,22 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 					? RWSEM_WAKE_READERS
 					: RWSEM_WAKE_ANY, &wake_q);
 
-		/*
-		 * The wakeup is normally called _after_ the wait_lock
-		 * is released, but given that we are proactively waking
-		 * readers we can deal with the wake_q overhead as it is
-		 * similar to releasing and taking the wait_lock again
-		 * for attempting rwsem_try_write_lock().
-		 */
-		wake_up_q(&wake_q);
-
-		/* We need wake_q again below, reinitialize */
-		wake_q_init(&wake_q);
+		if (!wake_q_empty(&wake_q)) {
+			/*
+			 * We want to minimize wait_lock hold time especially
+			 * when a large number of readers are to be woken up.
+			 */
+			raw_spin_unlock_irq(&sem->wait_lock);
+			wake_up_q(&wake_q);
+			wake_q_init(&wake_q);	/* Used again, reinit */
+			raw_spin_lock_irq(&sem->wait_lock);
+			/*
+			 * This waiter may have become first in the wait
+			 * list after re-acquring the wait_lock. The
+			 * rwsem_first_waiter() test in the main while
+			 * loop below will correctly detect that.
+			 */
+		}
 	} else {
 		count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count);
 	}
-- 
2.18.1

