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