linux-kernel - Re: [PATCH tip/locking/core v10 6/7] locking/pvqspinlock: Allow limited lock stealing

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Date:	Tue, 10 Nov 2015 17:03:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH tip/locking/core v10 6/7] locking/pvqspinlock: Allow
 limited lock stealing

On Mon, Nov 09, 2015 at 07:09:26PM -0500, Waiman Long wrote:
> @@ -291,7 +292,7 @@ static __always_inline void __pv_wait_head(struct qspinlock *lock,
>  void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  {
>  	struct mcs_spinlock *prev, *next, *node;
> -	u32 new, old, tail;
> +	u32 new, old, tail, locked;
>  	int idx;
>  
>  	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
> @@ -431,11 +432,25 @@ queue:
>  	 * sequentiality; this is because the set_locked() function below
>  	 * does not imply a full barrier.
>  	 *
> +	 * The PV pv_wait_head_or_lock function, if active, will acquire
> +	 * the lock and return a non-zero value. So we have to skip the
> +	 * smp_load_acquire() call. As the next PV queue head hasn't been
> +	 * designated yet, there is no way for the locked value to become
> +	 * _Q_SLOW_VAL. So both the set_locked() and the
> +	 * atomic_cmpxchg_relaxed() calls will be safe.
> +	 *
> +	 * If PV isn't active, 0 will be returned instead.
> +	 *
>  	 */
> -	pv_wait_head(lock, node);
> -	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
> +	locked = val = pv_wait_head_or_lock(lock, node);
> +	if (locked)
> +		goto reset_tail_or_wait_next;
> +
> +	while ((val = smp_load_acquire(&lock->val.counter))
> +			& _Q_LOCKED_PENDING_MASK)
>  		cpu_relax();
>  
> +reset_tail_or_wait_next:
>  	/*
>  	 * claim the lock:
>  	 *
> @@ -447,8 +462,12 @@ queue:
>  	 * to grab the lock.
>  	 */
>  	for (;;) {
> -		if (val != tail) {
> -			set_locked(lock);
> +		/*
> +		 * The lock value may or may not have the _Q_LOCKED_VAL bit set.
> +		 */
> +		if ((val & _Q_TAIL_MASK) != tail) {
> +			if (!locked)
> +				set_locked(lock);
>  			break;
>  		}
>  		/*

How about this instead? If we've already got _Q_LOCKED_VAL set, issuing
that store again isn't much of a problem, the cacheline is already hot
and we own it and its a regular store not an atomic.

@@ -432,10 +433,13 @@ void queued_spin_lock_slowpath(struct qs
 	 * does not imply a full barrier.
 	 *
 	 */
-	pv_wait_head(lock, node);
+	if ((val = pv_wait_head_or_lock(lock, node)))
+		goto locked;
+
 	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
 		cpu_relax();
 
+locked:
 	/*
 	 * claim the lock:
 	 *
@@ -447,7 +451,8 @@ void queued_spin_lock_slowpath(struct qs
 	 * to grab the lock.
 	 */
 	for (;;) {
-		if (val != tail) {
+		/* In the PV case we might already have _Q_LOCKED_VAL set */
+		if ((val & _Q_TAIL_MASK) != tail) {
 			set_locked(lock);
 			break;
 		}
--
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