linux-kernel - Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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Date:	Mon, 14 Sep 2015 15:19:41 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

On 09/14/2015 10:04 AM, Peter Zijlstra wrote:
> On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
>> This patch allows one attempt for the lock waiter to steal the lock
>> when entering the PV slowpath.  This helps to reduce the performance
>> penalty caused by lock waiter preemption while not having much of
>> the downsides of a real unfair lock.
>
>> @@ -416,7 +414,8 @@ queue:
>>   	 * does not imply a full barrier.
>>   	 *
>>   	 */
> If it really were once, like the Changelog says it is, then you could
> have simply added:
>
> 	if (pv_try_steal_lock(...))
> 		goto release;

My previous mail has clarified where the lock stealing happen. Will add 
the necessary comment to the patch.

> here, and not wrecked pv_wait_head() like you did. Note that if you do
> it like this, you also do not need to play games with the hash, because
> you'll never get into that situation.
>
>> -	pv_wait_head(lock, node);
>> +	if (pv_wait_head_and_lock(lock, node, tail))
>> +		goto release;
>>   	while ((val = smp_load_acquire(&lock->val.counter))&  _Q_LOCKED_PENDING_MASK)
>>   		cpu_relax();
>>

Because we need to use atomic op to get the lock, we can't use the 
native logic to do the acquire. I know it is kind of hacky, but I don't 
have a good alternative here.

Cheers,
Longman
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