linux-kernel - Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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Message-ID: <5640F9B9.7090306@hpe.com>
Date:	Mon, 09 Nov 2015 14:53:29 -0500
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 tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1
 lock stealing attempt

On 11/09/2015 12:29 PM, Peter Zijlstra wrote:
> On Fri, Nov 06, 2015 at 12:47:49PM -0500, Waiman Long wrote:
>> On 11/06/2015 09:50 AM, Peter Zijlstra wrote:
>>> *urgh*, last time we had:
>>>
>>> +	if (pv_wait_head_or_steal())
>>> +		goto stolen;
>>> 	while ((val = smp_load_acquire(&lock->val.counter))&   _Q_LOCKED_PENDING_MASK)
>>> 		cpu_relax();
>>>
>>> 	...
>>>
>>> +stolen:
>>> 	while (!(next = READ_ONCE(node->next)))
>>> 		cpu_relax();
>>>
>>> 	...
>>>
>>> Now you completely overhaul the native code.. what happened?
>> I want to reuse as much of the existing native code as possible instead of
>> duplicating that in the PV function. The only difference now is that the PV
>> function will acquire that lock.
> Right; and while I doubt it hurts the native case (you did benchmark it
> I hope), I'm not too keen on the end result code wise.
>
> Maybe just keep the above.

I can jump over the smp_load_acquire() for PV instead of adding an 
additional if block. For the native code, the only thing that was added 
was an additional masking of val with _Q_TAIL_MASK which I don't think 
will make too much of a difference.
>
>> Semantically, I don't want to call the lock
>> acquisition as lock stealing as the queue head is entitled to get the lock
>> next.
> Fair enough I suppose, pv_wait_head_or_lock() then?
>

I am fine with that name.

>> I can rename pv_queued_spin_trylock_unfair() to
>> pv_queued_spin_steal_lock() to emphasize the fact that this is the routine
>> where lock stealing happens.
> OK.
>

Cheers,
Longman
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