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

[Peter Zijlstra]

On Fri, Oct 30, 2015 at 07:26:33PM -0400, Waiman Long wrote:
> A queue head CPU, after acquiring the lock, will have to notify
> the next CPU in the wait queue that it has became the new queue
> head. This involves loading a new cacheline from the MCS node of the
> next CPU. That operation can be expensive and add to the latency of
> locking operation.
> 
> This patch addes code to optmistically prefetch the next MCS node
> cacheline if the next pointer is defined and it has been spinning
> for the MCS lock for a while. This reduces the locking latency and
> improves the system throughput.
> 
> Using a locking microbenchmark on a Haswell-EX system, this patch
> can improve throughput by about 5%.

How does it affect IVB-EX (which you were testing earlier IIRC)?

> Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
> ---
>  kernel/locking/qspinlock.c |   21 +++++++++++++++++++++
>  1 files changed, 21 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 7868418..c1c8a1a 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -396,6 +396,7 @@ queue:
>  	 * p,*,* -> n,*,*
>  	 */
>  	old = xchg_tail(lock, tail);
> +	next = NULL;
>  
>  	/*
>  	 * if there was a previous node; link it and wait until reaching the
> @@ -407,6 +408,16 @@ queue:
>  
>  		pv_wait_node(node);
>  		arch_mcs_spin_lock_contended(&node->locked);
> +
> +		/*
> +		 * While waiting for the MCS lock, the next pointer may have
> +		 * been set by another lock waiter. We optimistically load
> +		 * the next pointer & prefetch the cacheline for writing
> +		 * to reduce latency in the upcoming MCS unlock operation.
> +		 */
> +		next = READ_ONCE(node->next);
> +		if (next)
> +			prefetchw(next);
>  	}

OK so far I suppose. Since we already read node->locked, which is in the
same cacheline, also reading node->next isn't extra pressure. And we can
then prefetch that cacheline.

>  	/*
> @@ -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?
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