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

[Kyle Moffett]

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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