Messages in this thread | | | Date | Fri, 18 Apr 2014 14:13:57 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word |
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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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