Re: [RFC PATCH-tip v2 3/6] locking/rwsem: Enable count-based spinning on reader

Peter Zijlstra
2016-06-15T17:38:30Z

On Tue, Jun 14, 2016 at 06:48:06PM -0400, Waiman Long wrote:
> static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> {
> - bool taken = false;
> + bool taken = false, can_spin;

I would place the variables without assignment first.

> + int loopcnt;
>
> preempt_disable();
>
> @@ -409,6 +412,8 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> if (!osq_lock(&sem->osq))
> goto done;
>
> + loopcnt = sem->rspin_enabled ? RWSEM_RSPIN_THRESHOLD : 0;
> +
> /*
> * Optimistically spin on the owner field and attempt to acquire the
> * lock whenever the owner changes. Spinning will be stopped when:
> @@ -416,7 +421,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> * 2) readers own the lock as we can't determine if they are
> * actively running or not.
> */
> - while (rwsem_spin_on_owner(sem)) {
> + while ((can_spin = rwsem_spin_on_owner(sem)) || loopcnt) {
> /*
> * Try to acquire the lock
> */
> @@ -425,13 +430,16 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> break;
> }
>
> + if (!can_spin && loopcnt)
> + loopcnt--;

This seems to suggest 'can_spin' is a bad name, because if we cannot
spin, we do in fact spin anyway?

Maybe call it write_spin or something, which makes it clear that if its
not a write spin we'll do a read spin?

Also, isn't this the wrong level to do loopcnt at?
rwsem_spin_on_owner() can have spend any amount of cycles spinning. So
you're not counting loops of similar unit.

> + /*
> + * Was owner a reader?
> + */
> + if (rwsem_owner_is_reader(sem->owner)) {
> + /*
> + * Update rspin_enabled for reader spinning

full stop and newline?

> + * Increment by 1 if successfully & decrement by 8 if
> + * unsuccessful.

This is bloody obvious from the code, explain why, not what the code
does.

> The decrement amount is kind of arbitrary
> + * and can be adjusted if necessary.
> + */
> + if (taken && (sem->rspin_enabled < RWSEM_RSPIN_ENABLED_MAX))
> + sem->rspin_enabled++;
> + else if (!taken)
> + sem->rspin_enabled = (sem->rspin_enabled >= 8)
> + ? sem->rspin_enabled - 8 : 0;

This is unreadable and against coding style.

> + }
> osq_unlock(&sem->osq);
> done:
> preempt_enable();