linux-kernel - Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word

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Message-ID: <535161BF.90405@hp.com>
Date:	Fri, 18 Apr 2014 13:32:47 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail
 code word

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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