On May 29, 2005, at 04:42:43, Nikita Danilov wrote:
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.
Absolutely. I agree on all points (And that's why I added the function
spinaphore_lock_atomic() so that they could be nested a little bit.
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?
Oops, agreed, see other email for a fix.
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.
The reason for the atomic reads there is so that the value is updated,
instead of cached in a register.
In an assembly implementation, this would be optimized such that it all
fits in one cacheline and uses a minimum number of atomic operations and
cache flushes. This is a naive C implementation based on existing
primitives.
Cheers,
Kyle Moffett
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