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SubjectRe: spinaphore conceptual draft (was discussion of RT patch)
Kyle Moffett writes:

[...]

>
> struct spinaphore {
> atomic_t queued;
> atomic_t hold_time;
> spinlock_t spinlock;
> unsigned long acquire_time;
> };
>
> void spinaphore_lock (struct spinaphore *sph) {
> unsigned long start_time = fast_monotonic_count();

fast_monotonic_count() should be per-cpu, otherwise spinaphore_lock()
would require two atomic operations in the best case (and be twice as
expensive as a spin-lock). Per-cpu counter is OK, as long as thread is
not allowed to schedule with spinaphore held.

> int queue_me = 1;
> until (likely(spin_trylock(&sph->spinlock))) {
>
> /* Get the queue count (And ensure we're queued in the
> process) */
> unsigned int queued = queue_me ?
> atomic_inc_return(&sph->queued) :
> queued = atomic_get(&sph->queued);
> queue_me = 0;
>
> /* Figure out if we should switch away */
> if (unlikely(CONFIG_SPINAPHORE_CONTEXT_SWITCH <
> ( queued*atomic_get(&sph->hold_time) -
> fast_monotonic_count() - start_time
> ))) {
> /* Remove ourselves from the wait pool (remember to re-
> add later) */
> atomic_dec(&sph->queued);
> queue_me = 1;
>
> /* Go to sleep */
> cond_resched();
> }
> }
>
> /* Dequeue ourselves and update the acquire time */
> atomic_dec(&sph->queued);

atomic_dec() should only be done if atomic_inc_return() above was, i.e.,
not in contentionless fast-path, right?

[...]

>
> void spinaphore_unlock (struct spinaphore *sph) {
> /* Update the running average hold time */
> atomic_set(&sph->hold_time, (4*atomic_get(&sph->hold_time) +
> (fast_monotonic_count() - sph->acquire_time))/5);
>
> /* Actually unlock the spinlock */
> spin_unlock(&sph->spinlock);
> }

It is not good that unlock requires additional atomic operation. Why
->hold_time is atomic in the first place? It is only updated by the lock
holder, and as it is approximate statistics anyway, non-atomic reads in
spinaphore_lock() would be fine.

>
> Cheers,
> Kyle Moffett

Nikita.
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