Re: [Xen-devel] [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, Apr 18, 2014 at 01:32:47PM -0400, Waiman Long wrote:
> On 04/18/2014 04:15 AM, Peter Zijlstra wrote:
> >On Thu, Apr 17, 2014 at 05:28:17PM -0400, Waiman Long wrote:
> >>On 04/17/2014 11:49 AM, Peter Zijlstra wrote:
> >>>On Thu, Apr 17, 2014 at 11:03:56AM -0400, Waiman Long wrote:
> >>>>@@ -192,36 +220,25 @@ void queue_spin_lock_slowpath(struct qspinlock 
> >>>>*lock, u32 val)
> >>>>          node->next = NULL;
> >>>>
> >>>>          /*
> >>>>+  * We touched a (possibly) cold cacheline; attempt the trylock once
> >>>>+  * more in the hope someone let go while we weren't watching as long
> >>>>+  * as no one was queuing.
> >>>>           */
> >>>>+ if (!(val&   _Q_TAIL_MASK)&&   queue_spin_trylock(lock))
> >>>>+         goto release;
> >>>But you just did a potentially very expensive op; @val isn't
> >>>representative anymore!
> >>That is not true. I pass in a pointer to val to trylock_pending() (the
> >>pointer thing) so that it will store the latest value that it reads from the
> >>lock back into val. I did miss one in the PV qspinlock exit loop. I will add
> >>it back when I do the next version.
> >But you did that read _before_ you touched a cold cacheline, that's 100s
> >of cycles. Whatever value you read back then is now complete nonsense.
> 
> For spin_lock(), the lock cacheline is touched by a cmpxchg(). It can takes
> 100s of cycles whether it is hot or cold.

Its not the lock cacheline, you just touched the per-cpu node cacheline
for the first time, setting up the node.


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