Re: [PATCH v11 09/16] qspinlock, x86: Allow unfair spinlock in a virtual guest

[Peter Zijlstra]

On Wed, Jun 11, 2014 at 09:37:55PM -0400, Long, Wai Man wrote:
> 
> On 6/11/2014 6:54 AM, Peter Zijlstra wrote:
> >On Fri, May 30, 2014 at 11:43:55AM -0400, Waiman Long wrote:
> >>Enabling this configuration feature causes a slight decrease the
> >>performance of an uncontended lock-unlock operation by about 1-2%
> >>mainly due to the use of a static key. However, uncontended lock-unlock
> >>operation are really just a tiny percentage of a real workload. So
> >>there should no noticeable change in application performance.
> >No, entirely unacceptable.
> >
> >>+#ifdef CONFIG_VIRT_UNFAIR_LOCKS
> >>+/**
> >>+ * queue_spin_trylock_unfair - try to acquire the queue spinlock unfairly
> >>+ * @lock : Pointer to queue spinlock structure
> >>+ * Return: 1 if lock acquired, 0 if failed
> >>+ */
> >>+static __always_inline int queue_spin_trylock_unfair(struct qspinlock *lock)
> >>+{
> >>+	union arch_qspinlock *qlock = (union arch_qspinlock *)lock;
> >>+
> >>+	if (!qlock->locked && (cmpxchg(&qlock->locked, 0, _Q_LOCKED_VAL) == 0))
> >>+		return 1;
> >>+	return 0;
> >>+}
> >>+
> >>+/**
> >>+ * queue_spin_lock_unfair - acquire a queue spinlock unfairly
> >>+ * @lock: Pointer to queue spinlock structure
> >>+ */
> >>+static __always_inline void queue_spin_lock_unfair(struct qspinlock *lock)
> >>+{
> >>+	union arch_qspinlock *qlock = (union arch_qspinlock *)lock;
> >>+
> >>+	if (likely(cmpxchg(&qlock->locked, 0, _Q_LOCKED_VAL) == 0))
> >>+		return;
> >>+	/*
> >>+	 * Since the lock is now unfair, we should not activate the 2-task
> >>+	 * pending bit spinning code path which disallows lock stealing.
> >>+	 */
> >>+	queue_spin_lock_slowpath(lock, -1);
> >>+}
> >Why is this needed?
> 
> I added the unfair version of lock and trylock as my original version isn't
> a simple test-and-set lock. Now I changed the core part to use the simple
> test-and-set lock. However, I still think that an unfair version in the fast
> path can be helpful to performance when both the unfair lock and paravirt
> spinlock are enabled. In this case, paravirt spinlock code will disable the
> unfair lock code in the slowpath, but still allow the unfair version in the
> fast path to get the best possible performance in a virtual guest.
> 
> Yes, I could take that out to allow either unfair or paravirt spinlock, but
> not both. I do think that a little bit of unfairness will help in the
> virtual environment.

When will you learn to like simplicity and stop this massive over
engineering effort?

There's no sane reason to have the test-and-set virt and paravirt locks
enabled at the same bloody time.

There's 3 distinct cases:

 - native
 - virt
 - paravirt

And they do not overlap. Furthermore, if there is any possibility at all
of not polluting the native code, grab it with both hands.

Native performance is king, try your very utmost bestest to preserve
that, paravirt is a distant second and nobody sane should care about the
virt case at all.

If you want extra lock stealing in the paravirt case, put it in the
slowpath code before you start queueing.

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