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Re: [Xen-devel] [PATCH v5 3/8] qspinlock, x86: Add x86 specific optimization for 2 contending tasks

On 02/26/2014 11:20 AM, Peter Zijlstra wrote:
You don't happen to have a proper state diagram for this thing do you?

I suppose I'm going to have to make one; this is all getting a bit
unwieldy, and those xchg() + fixup things are hard to read.

I don't have a state diagram on hand, but I will add more comments to describe the 4 possible cases and how to handle them.

On Wed, Feb 26, 2014 at 10:14:23AM -0500, Waiman Long wrote:
+static inline int queue_spin_trylock_quick(struct qspinlock *lock, int qsval)
+       union arch_qspinlock *qlock = (union arch_qspinlock *)lock;
+       u16                  old;
+       /*
+        * Fall into the quick spinning code path only if no one is waiting
+        * or the lock is available.
+        */
+       if (unlikely((qsval != _QSPINLOCK_LOCKED)&&
+                    (qsval != _QSPINLOCK_WAITING)))
+               return 0;
+       old = xchg(&qlock->lock_wait, _QSPINLOCK_WAITING|_QSPINLOCK_LOCKED);
+       if (old == 0) {
+               /*
+                * Got the lock, can clear the waiting bit now
+                */
+               smp_u8_store_release(&qlock->wait, 0);

So we just did an atomic op, and now you're trying to optimize this
write. Why do you need a whole byte for that?

Surely a cmpxchg loop with the right atomic op can't be _that_ much
slower? Its far more readable and likely avoids that steal fail below as

At low contention level, atomic operations that requires a lock prefix are the major contributor to the total execution times. I saw estimate online that the time to execute a lock prefix instruction can easily be 50X longer than a regular instruction that can be pipelined. That is why I try to do it with as few lock prefix instructions as possible. If I have to do an atomic cmpxchg, it probably won't be faster than the regular qspinlock slowpath.

Given that speed at low contention level which is the common case is important to get this patch accepted, I have to do what I can to make it run as far as possible for this 2 contending task case.

+               return 1;
+       } else if (old == _QSPINLOCK_LOCKED) {
+               /*
+                * Wait until the lock byte is cleared to get the lock
+                */
+               do {
+                       cpu_relax();
+               } while (ACCESS_ONCE(qlock->lock));
+               /*
+                * Set the lock bit&  clear the waiting bit
+                */
+               if (cmpxchg(&qlock->lock_wait, _QSPINLOCK_WAITING,
+                          _QSPINLOCK_LOCKED) == _QSPINLOCK_WAITING)
+                       return 1;
+               /*
+                * Someone has steal the lock, so wait again
+                */
+               goto try_again;
That's just a fail.. steals should not ever be allowed. It's a fair lock
after all.

The code is unfair, but this unfairness help it to run faster than ticket spinlock in this particular case. And the regular qspinlock slowpath is fair. A little bit of unfairness in this particular case helps its speed.

+       } else if (old == _QSPINLOCK_WAITING) {
+               /*
+                * Another task is already waiting while it steals the lock.
+                * A bit of unfairness here won't change the big picture.
+                * So just take the lock and return.
+                */
+               return 1;
+       }
+       /*
+        * Nothing need to be done if the old value is
+        */
+       return 0;

@@ -296,6 +478,9 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, int 

+#ifdef queue_code_xchg
+       prev_qcode = queue_code_xchg(lock, my_qcode);
         * Exchange current copy of the queue node code
@@ -329,6 +514,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, int 
        } else
                prev_qcode&= ~_QSPINLOCK_LOCKED;    /* Clear the lock bit */
        my_qcode&= ~_QSPINLOCK_LOCKED;
+#endif /* queue_code_xchg */

        if (prev_qcode) {
That's just horrible.. please just make the entire #else branch another
version of that same queue_code_xchg() function.

OK, I will wrap it in another function.


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