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 <stable@xxxxxxxxxxxxxxx> know about it.
>From 43b3f02899f74ae9914a39547cc5492156f0027a Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri, 4 Sep 2015 17:25:23 +0200
Subject: locking/qspinlock/x86: Fix performance regression under unaccelerated VMs
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
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.
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>
Link: http://lkml.kernel.org/r/20150904152523.GR18673@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
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;
/*
Patches currently in stable-queue which might be from peterz@xxxxxxxxxxxxx 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
--
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