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    SubjectRe: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word
    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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