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Re: [Xen-devel] [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word
- To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- From: Waiman Long <waiman.long@xxxxxx>
- Date: Fri, 18 Apr 2014 13:32:47 -0400
- Cc: linux-arch@xxxxxxxxxxxxxxx, Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>, Gleb Natapov <gleb@xxxxxxxxxx>, kvm@xxxxxxxxxxxxxxx, Scott J Norton <scott.norton@xxxxxx>, x86@xxxxxxxxxx, Paolo Bonzini <paolo.bonzini@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, Ingo Molnar <mingo@xxxxxxxxxx>, Chegu Vinod <chegu_vinod@xxxxxx>, David Vrabel <david.vrabel@xxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Oleg Nesterov <oleg@xxxxxxxxxx>
- Delivery-date: Fri, 18 Apr 2014 17:33:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
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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