Re: [PATCH tip/locking/core v9 2/6] locking/qspinlock: prefetch next node cacheline

[Waiman Long <waiman.long@hpe.com>]

On 11/05/2015 11:39 AM, Peter Zijlstra wrote:
> On Thu, Nov 05, 2015 at 11:06:48AM -0500, Waiman Long wrote:
>
>>> How does it affect IVB-EX (which you were testing earlier IIRC)?
>> My testing on IVB-EX indicated that if the critical section is really short,
>> the change may actually slow thing a bit in some cases. However, when the
>> critical section is long enough that the prefetch overhead can be hidden
>> within the lock acquisition loop, there will be a performance boost.
>>>> @@ -426,6 +437,15 @@ queue:
>>>>   		cpu_relax();
>>>>
>>>>   	/*
>>>> +	 * If the next pointer is defined, we are not tail anymore.
>>>> +	 * In this case, claim the spinlock&   release the MCS lock.
>>>> +	 */
>>>> +	if (next) {
>>>> +		set_locked(lock);
>>>> +		goto mcs_unlock;
>>>> +	}
>>>> +
>>>> +	/*
>>>>   	 * claim the lock:
>>>>   	 *
>>>>   	 * n,0,0 ->   0,0,1 : lock, uncontended
>>>> @@ -458,6 +478,7 @@ queue:
>>>>   	while (!(next = READ_ONCE(node->next)))
>>>>   		cpu_relax();
>>>>
>>>> +mcs_unlock:
>>>>   	arch_mcs_spin_unlock_contended(&next->locked);
>>>>   	pv_kick_node(lock, next);
>>>>
>>> This however appears an independent optimization. Is it worth it? Would
>>> we not already have observed a val != tail in this case? At which point
>>> we're just adding extra code for no gain.
>>>
>>> That is, if we observe @next, must we then not also observe val != tail?
>> Observing next implies val != tail, but the reverse may not be true. The
>> branch is done before we observe val != tail. Yes, it is an optimization to
>> avoid reading node->next again if we have already observed next. I have
>> observed a very minor performance boost with that change without the
>> prefetch.
> This is all good information to have in the Changelog. And since these
> are two independent changes, two patches would have been the right
> format.

Yep, I will separate it into 2 patches and include additional 
information in the changelog.

Cheers,
Longman