Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

[Peter Zijlstra]

On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
> This patch allows one attempt for the lock waiter to steal the lock
> when entering the PV slowpath.  This helps to reduce the performance
> penalty caused by lock waiter preemption while not having much of
> the downsides of a real unfair lock.
> 

Changelog does not explain the implementation, which is subtle enough to
warrant a few words.


> @@ -417,7 +415,8 @@ queue:
>  	 * does not imply a full barrier.
>  	 *
>  	 */
> -	pv_wait_head(lock, node);
> +	if (pv_wait_head_and_lock(lock, node, tail))
> +		goto release;

That's very much: pv_wait_head_or_lock(), maybe _or_steal() is even
better.

>  	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
>  		cpu_relax();
>  
> @@ -454,7 +453,6 @@ queue:
>  		cpu_relax();
>  
>  	arch_mcs_spin_unlock_contended(&next->locked);
> -	pv_kick_node(lock, next);

Not sure about removing that, breaks symmetry.

>  /*
> + * Allow one unfair trylock when entering the PV slowpath to reduce the
> + * performance impact of lock waiter preemption (either explicitly via
> + * pv_wait or implicitly via PLE). This function will be called once when
> + * a lock waiter enter the slowpath before being queued.
> + *
> + * A little bit of unfairness here can improve performance without many
> + * of the downsides of a real unfair lock.
> + */
> +#define queued_spin_trylock(l)	pv_queued_spin_trylock_unfair(l)
> +static inline bool pv_queued_spin_trylock_unfair(struct qspinlock *lock)
> +{
> +	struct __qspinlock *l = (void *)lock;
> +
> +	if (READ_ONCE(l->locked))
> +		return 0;
> +	/*
> +	 * Wait a bit here to ensure that an actively spinning queue head vCPU
> +	 * has a fair chance of getting the lock.
> +	 */
> +	cpu_relax();
> +
> +	return cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0;
> +}

This doesn't seem to make any sense.. Its also very much distinct from
the rest of the patch and can easily be added in a separate patch with
separate performance numbers to show it does (or does not) make a
difference.
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