On 04/18/2014 01:53 PM, Peter Zijlstra wrote:
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.
Thank for the clarification, now I know what you mean.
-Longman
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