Message-ID: <5640F9B9.7090306@hpe.com>
Date: Mon, 09 Nov 2015 14:53:29 -0500
From: Waiman Long <waiman.long@hpe.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH tip/locking/core v9 5/6] locking/pvqspinlock: Allow 1
 lock stealing attempt
References: <1446247597-61863-1-git-send-email-Waiman.Long@hpe.com> <1446247597-61863-6-git-send-email-Waiman.Long@hpe.com> <20151106145005.GU17308@twins.programming.kicks-ass.net> <563CE7C5.9020508@hpe.com> <20151109172945.GM17308@twins.programming.kicks-ass.net>
In-Reply-To: <20151109172945.GM17308@twins.programming.kicks-ass.net>
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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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