Patch "locking/qspinlock/x86: Fix performance regression under unaccelerated VMs" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

locking/qspinlock/x86: Fix performance regression under unaccelerated VMs

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
locking-qspinlock-x86-fix-performance-regression-under-unaccelerated-vms.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Peter Zijlstra <[email protected]>
Date: Fri, 4 Sep 2015 17:25:23 +0200
Subject: locking/qspinlock/x86: Fix performance regression under unaccelerated VMs

From: Peter Zijlstra <[email protected]>

commit 43b3f02899f74ae9914a39547cc5492156f0027a upstream.

Dave ran into horrible performance on a VM without PARAVIRT_SPINLOCKS set and Linus noted that the test-and-set implementation was retarded.

One should spin on the variable with a load, not a RMW.

While there, remove 'queued' from the name, as the lock isn't queued at all, but a simple test-and-set.

index 9d51fae1cba3..8dde3bdc4a05 100644
@@ -39,15 +39,23 @@ static inline void queued_spin_unlock(struct qspinlock *lock)
 }
 #endif
 
-#define virt_queued_spin_lock virt_queued_spin_lock
+#define virt_spin_lock virt_spin_lock
 
-static inline bool virt_queued_spin_lock(struct qspinlock *lock)
+static inline bool virt_spin_lock(struct qspinlock *lock)
 {
 	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
 		return false;
 
-	while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0)
-		cpu_relax();
+	/*
+	 * On hypervisors without PARAVIRT_SPINLOCKS support we fall
+	 * back to a Test-and-Set spinlock, because fair locks have
+	 * horrible lock 'holder' preemption issues.
+	 */
+
+	do {
+		while (atomic_read(&lock->val) != 0)
+			cpu_relax();
+	} while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
 
 	return true;
 }

index 83bfb87f5bf1..e2aadbc7151f 100644
@@ -111,8 +111,8 @@ static inline void queued_spin_unlock_wait(struct qspinlock *lock)
 		cpu_relax();
 }
 
-#ifndef virt_queued_spin_lock
-static __always_inline bool virt_queued_spin_lock(struct qspinlock *lock)
+#ifndef virt_spin_lock
+static __always_inline bool virt_spin_lock(struct qspinlock *lock)
 {
 	return false;
 }

index 337c8818541d..87e9ce6a63c5 100644
@@ -289,7 +289,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	if (pv_enabled())
 		goto queue;
 
-	if (virt_queued_spin_lock(lock))
+	if (virt_spin_lock(lock))
 		return;
 
 	/*

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/time-fix-timekeeping_freqadjust-s-incorrect-use-of-abs-instead-of-abs64.patch queue-4.2/perf-x86-intel-fix-constraint-access.patch queue-4.2/sched-access-local-runqueue-directly-in-single_task_running.patch queue-4.2/locking-qspinlock-x86-only-emit-the-test-and-set-fallback-when-building-guest-support.patch queue-4.2/locking-qspinlock-x86-fix-performance-regression-under-unaccelerated-vms.patch