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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