[tip:locking/core] locking/qspinlock/x86: Fix performance regression under unaccelerated VMs

[tip-bot for Peter Zijlstra]

Commit-ID:  43b3f02899f74ae9914a39547cc5492156f0027a
Gitweb:     http://git.kernel.org/tip/43b3f02899f74ae9914a39547cc5492156f0027a
Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Fri, 4 Sep 2015 17:25:23 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 11 Sep 2015 07:49:42 +0200

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

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.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Reported-by: Dave Chinner <david@xxxxxxxxxxxxx>
Tested-by: Dave Chinner <david@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Waiman Long <Waiman.Long@xxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.2+
Link: http://lkml.kernel.org/r/20150904152523.GR18673@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/include/asm/qspinlock.h | 16 ++++++++++++----
 include/asm-generic/qspinlock.h  |  4 ++--
 kernel/locking/qspinlock.c       |  2 +-
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index 9d51fae..8dde3bd 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -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;
 }
diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index 83bfb87..e2aadbc 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -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;
 }
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 337c881..87e9ce6a 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -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;
 
 	/*
--
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