linux-kernel - Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Date:   Thu, 10 Aug 2017 12:22:19 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs

On 08/10/2017 12:15 PM, Peter Zijlstra wrote:
> On Thu, Aug 10, 2017 at 09:58:57AM -0400, Waiman Long wrote:
>> On 08/10/2017 09:27 AM, Waiman Long wrote:
>>> On 08/10/2017 07:50 AM, Peter Zijlstra wrote:
>>>> On Wed, May 24, 2017 at 09:38:28AM -0400, Waiman Long wrote:
>>>>>   # of thread     w/o patch    with patch      % Change
>>>>>   -----------     ---------    ----------      --------
>>>>>        4         4053.3 Mop/s  4223.7 Mop/s     +4.2%
>>>>>        8         3310.4 Mop/s  3406.0 Mop/s     +2.9%
>>>>>       12         2576.4 Mop/s  2674.6 Mop/s     +3.8%
>>>> Waiman, could you run those numbers again but with the below 'fixed' ?
>>>>
>>>>> @@ -361,6 +361,13 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
>>>>>  	 * observe its next->locked value and advance itself.
>>>>>  	 *
>>>>>  	 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
>>>>> +	 *
>>>>> +	 * The write to next->locked in arch_mcs_spin_unlock_contended()
>>>>> +	 * must be ordered before the read of pn->state in the cmpxchg()
>>>>> +	 * below for the code to work correctly. However, this is not
>>>>> +	 * guaranteed on all architectures when the cmpxchg() call fails.
>>>>> +	 * Both x86 and PPC can provide that guarantee, but other
>>>>> +	 * architectures not necessarily.
>>>>>  	 */
>>>> 	smp_mb();
>>>>
>>>>>  	if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
>>>>>  		return;
>>>> Ideally this Power CPU can optimize back-to-back SYNC instructions, but
>>>> who knows...
>>> Yes, I can run the numbers again. However, the changes here is in the
>>> slowpath. My current patch optimizes the fast path only and my original
>>> test doesn't stress the slowpath at all, I think. I will have to make
>>> some changes to stress the slowpath.
>> Looking at past emails, I remember why I put the comment there. Putting
>> an smp_mb() here will definitely has an negative performance impact on
>> x86. So I put in the comment here to remind me that the current code may
>> not work for ARM64.
>>
>> To fix that, my current thought is to have a cmpxchg variant that
>> guarantees ordering for both success and failure, for example,
>> cmpxchg_ordered(). In that way, we only need to insert the barrier for
>> architectures that need it. That will be a separate patch instead of
>> integrating into this one.
> Might as well do an explicit:
>
> 	smp_mb__before_atomic()
> 	cmpxchg_relaxed()
> 	smp_mb__after_atomic()
>
> I suppose and not introduce new primitives.


Right. I think that will work without impacting current x86 performance.
Will update my patch accordingly.

Thanks,
Longman