Messages in this thread | | | Date | Fri, 18 Apr 2014 13:32:47 -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 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.
I will take the precheck out, it is not such a big deal anyway.
-Longman
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