Re: [PATCH] locking/osq: Use more optimized spinning for arm64

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 09, 2020 at 10:38:31AM -0500, Waiman Long wrote:

> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -134,6 +134,27 @@ bool osq_lock(struct optimistic_spin_queue *lock)
>  	 * cmpxchg in an attempt to undo our queueing.
>  	 */
>  
> +	/*
> +	 * If vcpu_is_preempted is not defined, we can skip the check
> +	 * and use smp_cond_load_relaxed() instead. For arm64, this
> +	 * could lead to the use of the more optimized wfe instruction.
> +	 * As need_sched() is set by interrupt handler, it will break
> +	 * out and do the unqueue in a timely manner.
> +	 *
> +	 * TODO: We may need to add a static_key like vcpu_is_preemptible
> +	 *	 as vcpu_is_preempted() will always return false with
> +	 *	 bare metal even if it is defined.
> +	 */
> +#ifndef vcpu_is_preempted
> +	{
> +		int locked = smp_cond_load_relaxed(&node->locked,
> +						   VAL || need_resched());
> +		if (!locked)
> +			goto unqueue;
> +		return true;
> +	}
> +#endif

Much yuck :-/

With ARM64 being the only arch that currently makes use of this; another
approach is doing something like:

That is also rather yuck, and definitely needs a few comments sprinked
on it, but it should just work for everyone.

It basically relies on an arch having a spinning *cond_load*()
implementation if it has vcpu_is_preempted(), which is true today.

---
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 6ef600aa0f47..6e00d7c077ba 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -133,18 +133,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 * guaranteed their existence -- this allows us to apply
 	 * cmpxchg in an attempt to undo our queueing.
 	 */
+	if (!smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
+						  vcpu_is_preempetd(node_cpu(node->prev))))
+		goto unqueue;
 
-	while (!READ_ONCE(node->locked)) {
-		/*
-		 * If we need to reschedule bail... so we can block.
-		 * Use vcpu_is_preempted() to avoid waiting for a preempted
-		 * lock holder:
-		 */
-		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
-			goto unqueue;
-
-		cpu_relax();
-	}
 	return true;
 
 unqueue: