Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 04, 2015 at 05:14:27PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 04, 2015 at 08:05:16AM -0700, Linus Torvalds wrote:
> > So at the very *minimum*, that second issue should be fixed, and the
> > loop in virt_queued_spin_lock() should look something like
> > 
> >     do {
> >         while (READ_ONCE(lock->val) != 0)
> >             cpu_relax();
> >     } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
> > 
> > which at least has a chance in hell of behaving well on the bus and in
> > a HT environment.
> 
> True.

Something like so...

---
Subject: locking: Fix virt test-and-set lock implementation

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

Reported-by: Dave Chinner <david@fromorbit.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 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 9d51fae1cba3..8dde3bdc4a05 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 83bfb87f5bf1..e2aadbc7151f 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 337c8818541d..87e9ce6a63c5 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;
 
 	/*