[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word
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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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