linux-kernel - [PATCH 1/3] locking/rwlocks: Add contention detection for rwlocks

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Date:   Tue, 27 Oct 2020 09:49:48 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH 1/3] locking/rwlocks: Add contention detection for rwlocks

rwlocks do not currently have any facility to detect contention
like spinlocks do. In order to allow users of rwlocks to better manage
latency, add contention detection for queued rwlocks.

Signed-off-by: Ben Gardon <bgardon@...gle.com>
---
 include/asm-generic/qrwlock.h | 23 +++++++++++++++++------
 include/linux/rwlock.h        |  7 +++++++
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 3aefde23dceab..c4be4673a6ab2 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -116,15 +116,26 @@ static inline void queued_write_unlock(struct qrwlock *lock)
 	smp_store_release(&lock->wlocked, 0);
 }
 
+/**
+ * queued_rwlock_is_contended - check if the lock is contended
+ * @lock : Pointer to queue rwlock structure
+ * Return: 1 if lock contended, 0 otherwise
+ */
+static inline int queued_rwlock_is_contended(struct qrwlock *lock)
+{
+	return arch_spin_is_locked(&lock->wait_lock);
+}
+
 /*
  * Remapping rwlock architecture specific functions to the corresponding
  * queue rwlock functions.
  */
-#define arch_read_lock(l)	queued_read_lock(l)
-#define arch_write_lock(l)	queued_write_lock(l)
-#define arch_read_trylock(l)	queued_read_trylock(l)
-#define arch_write_trylock(l)	queued_write_trylock(l)
-#define arch_read_unlock(l)	queued_read_unlock(l)
-#define arch_write_unlock(l)	queued_write_unlock(l)
+#define arch_read_lock(l)		queued_read_lock(l)
+#define arch_write_lock(l)		queued_write_lock(l)
+#define arch_read_trylock(l)		queued_read_trylock(l)
+#define arch_write_trylock(l)		queued_write_trylock(l)
+#define arch_read_unlock(l)		queued_read_unlock(l)
+#define arch_write_unlock(l)		queued_write_unlock(l)
+#define arch_rwlock_is_contended(l)	queued_rwlock_is_contended(l)
 
 #endif /* __ASM_GENERIC_QRWLOCK_H */
diff --git a/include/linux/rwlock.h b/include/linux/rwlock.h
index 3dcd617e65ae9..7ce9a51ae5c04 100644
--- a/include/linux/rwlock.h
+++ b/include/linux/rwlock.h
@@ -128,4 +128,11 @@ do {								\
 	1 : ({ local_irq_restore(flags); 0; }); \
 })
 
+#ifdef arch_rwlock_is_contended
+#define rwlock_is_contended(lock) \
+	 arch_rwlock_is_contended(&(lock)->raw_lock)
+#else
+#define rwlock_is_contended(lock)	((void)(lock), 0)
+#endif /* arch_rwlock_is_contended */
+
 #endif /* __LINUX_RWLOCK_H */
-- 
2.29.0.rc2.309.g374f81d7ae-goog