Re: spinaphore conceptual draft (was discussion of RT patch)

[Nikita Danilov <nikita@clusterfs.com>]

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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