Re: [Xen-devel] [PATCH v10 06/19] qspinlock: prolong the stay in the pending bit path

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, May 09, 2014 at 08:58:47PM -0400, Waiman Long wrote:
> On 05/08/2014 02:58 PM, Peter Zijlstra wrote:
> >On Wed, May 07, 2014 at 11:01:34AM -0400, Waiman Long wrote:
> >>@@ -221,11 +222,37 @@ static inline int trylock_pending(struct qspinlock 
> >>*lock, u32 *pval)
> >>     */
> >>    for (;;) {
> >>            /*
> >>-            * If we observe any contention; queue.
> >>+            * If we observe that the queue is not empty,
> >>+            * return and be queued.
> >>             */
> >>-           if (val&  ~_Q_LOCKED_MASK)
> >>+           if (val&  _Q_TAIL_MASK)
> >>                    return 0;
> >>
> >>+           if (val == (_Q_LOCKED_VAL|_Q_PENDING_VAL)) {
> >>+                   /*
> >>+                    * If both the lock and pending bits are set, we wait
> >>+                    * a while to see if that either bit will be cleared.
> >>+                    * If that is no change, we return and be queued.
> >>+                    */
> >>+                   if (!retry)
> >>+                           return 0;
> >>+                   retry--;
> >>+                   cpu_relax();
> >>+                   cpu_relax();
> >>+                   *pval = val = atomic_read(&lock->val);
> >>+                   continue;
> >>+           } else if (val == _Q_PENDING_VAL) {
> >>+                   /*
> >>+                    * Pending bit is set, but not the lock bit.
> >>+                    * Assuming that the pending bit holder is going to
> >>+                    * set the lock bit and clear the pending bit soon,
> >>+                    * it is better to wait than to exit at this point.
> >>+                    */
> >>+                   cpu_relax();
> >>+                   *pval = val = atomic_read(&lock->val);
> >>+                   continue;
> >>+           }
> >Didn't I give a much saner alternative to this mess last time?
> 
> I don't recall you have any suggestion last time. Anyway, if you think the
> code is too messy, I think I can give up the first if statement which is
> more an optimistic spinning kind of code for short critical section. The 2nd
> if statement is still need to improve chance of using this code path due to
> timing reason. I will rerun my performance test to make sure it won't have
> too much performance impact.

lkml.kernel.org/r/20140417163640.GT11096@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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