Re: [RFC PATCH] qspinlock: Improve performance by reducing load instruction rollback

[Peter Zijlstra]

On Mon, Oct 19, 2015 at 10:27:22AM +0800, ling.ma.program@xxxxxxxxx wrote:
> From: Ma Ling <ling.ml@xxxxxxxxxxxxxxx>
> 
> All load instructions can run speculatively but they have to follow
> memory order rule in multiple cores as below:
> _x = _y = 0
> 
> Processor 0				Processor 1
> 
> mov r1, [ _y]  //M1			mov [ _x], 1  //M3
> mov r2, [ _x]  //M2			mov [ _y], 1  //M4
> 
> If r1 = 1, r2 must be 1
> 
> In order to guarantee above rule, although Processor 0 execute
> M1 and M2 instruction out of order, they are kept in ROB,
> when load buffer for _x in Processor 0 received the update 
> message from Processor 1, Processor 0 need to roll back
> from M2 instruction, which will flush the whole pipeline,
> the latency is over the penalty from branch prediction miss.
> 
> In this patch we use lock cmpxchg instruction to force load
> instructions to be serialization, the destination operand
> receives a write cycle without regard to the result of
> the comparison, which can help us to reduce the penalty
> from load instruction roll back.
> 
> Our experiment indicates the performance can be improved by 10%~15%
> for 2 and 3 threads cases, the conflicts from lock cache line
> spend them most of the time.

On what hardware? Also, you forgot to Cc Waiman, who is a prime author
of this code. Excessive quoting for his benefit.

> Signed-off-by: Ma Ling <ling.ml@xxxxxxxxxxxxxxx>
> ---
>  kernel/locking/qspinlock.c |   43 ++++++++++++++++++-------------------------
>  1 files changed, 18 insertions(+), 25 deletions(-)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 87e9ce6..16421f2 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -332,25 +332,14 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	if (new == _Q_LOCKED_VAL)
>  		return;
>  
> -	/*
> -	 * we're pending, wait for the owner to go away.
> -	 *
> -	 * *,1,1 -> *,1,0
> +	/* we're waiting, and get lock owner

That's incorrect coding style

>  	 *
> -	 * this wait loop must be a load-acquire such that we match the
> -	 * store-release that clears the locked bit and create lock
> -	 * sequentiality; this is because not all clear_pending_set_locked()
> -	 * implementations imply full barriers.
> +	 * *,1,* -> *,0,1
>  	 */
> -	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_MASK)
> +	while (cmpxchg(&((struct __qspinlock *)lock)->locked_pending,
> +		_Q_PENDING_VAL, _Q_LOCKED_VAL) != _Q_PENDING_VAL)

That's both horrible coding style and painful, we should not spin-wait
with a cmpxchg instruction like that.

>  		cpu_relax();
> -
> -	/*
> -	 * take ownership and clear the pending bit.
> -	 *
> -	 * *,1,0 -> *,0,1
> -	 */
> -	clear_pending_set_locked(lock);
> +
>  	return;
>  
>  	/*
> @@ -399,17 +388,21 @@ queue:
>  	 * we're at the head of the waitqueue, wait for the owner & pending to
>  	 * go away.
>  	 *
> -	 * *,x,y -> *,0,0
> -	 *
> -	 * this wait loop must use a load-acquire such that we match the
> -	 * store-release that clears the locked bit and create lock
> -	 * sequentiality; this is because the set_locked() function below
> -	 * does not imply a full barrier.
> -	 *
> +	 * *,x,y -> *,0,1
>  	 */
>  	pv_wait_head(lock, node);
> -	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
> +	next = READ_ONCE(node->next);
> +	while (cmpxchg(&((struct __qspinlock *)lock)->locked_pending, 0,
> +		_Q_LOCKED_VAL) != 0) {

idem

> +		next = READ_ONCE(node->next);
>  		cpu_relax();
> +	}
> +
> +	if (next)
> +		goto next_node;
> +
> +	val = smp_load_acquire(&lock->val.counter);
> +	tail = tail | _Q_LOCKED_VAL;
>  
>  	/*
>  	 * claim the lock:
> @@ -423,7 +416,6 @@ queue:
>  	 */
>  	for (;;) {
>  		if (val != tail) {
> -			set_locked(lock);
>  			break;
>  		}
>  		old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL);
> @@ -439,6 +431,7 @@ queue:
>  	while (!(next = READ_ONCE(node->next)))
>  		cpu_relax();
>  
> +next_node:
>  	arch_mcs_spin_unlock_contended(&next->locked);
>  	pv_kick_node(lock, next);
>  
> -- 
> 1.7.1
> 
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