linux-kernel - Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

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Message-ID: <561D6E91.90703@hpe.com>
Date:	Tue, 13 Oct 2015 16:50:25 -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 v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

On 10/13/2015 03:56 PM, Peter Zijlstra wrote:
> On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
>
>> +gotlock:
>>   	/*
>> +	 * We now have the lock. We need to either clear the tail code or
>> +	 * notify the next one in queue as the new queue head.
>>   	 */
>> +	old = atomic_read(&lock->val);
>> +	while ((old&  _Q_TAIL_MASK) == tail) {
>> +		int val;
>> +		int new = old&  ~_Q_TAIL_MASK;
>> +
>> +		/*
>> +		 * We are the only one in the queue, so clear the tail code
>> +		 * and return.
>> +		 */
>> +		val = atomic_cmpxchg(&lock->val, old, new);
>> +		if (old == val)
>> +			goto done;
>> +		old = val;
>> +	}
>> +
> This i need to think about a wee bit; its almost the same...
>
>
> So the below is exactly duplicated from the normal slowpath, so why
> don't you keep that there?
>
> It would get you something like:
>
> 	if (pv_wait_head_or_steal(..))
> 		goto stolen;
>
>
> stolen:
>> +	/*
>> +	 * contended path; wait for next, release.
>> +	 */
>> +	while (!(next = READ_ONCE(node->next)))
>> +		cpu_relax();
>> +
>> +	arch_mcs_spin_unlock_contended(&next->locked);
>> +	pv_kick_node(lock, next);
> release:
> 	...

Yes, it is largely the same. I thought that you don't like too much 
change in the logic flow of the generic qspinlock code. I will make the 
change in the next revision.

Cheers,
Longman
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