Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

Peter Zijlstra
2009-01-09T15:07:47Z

On Fri, 2009-01-09 at 11:47 +0100, Peter Zijlstra wrote:

> > So I think the bug is still there, we just hid it better by breaking out
> > of the loop with that "if (need_resched())" always eventually triggering.
> > And it would be ok if it really is guaranteed to _eventually_ trigger, and
> > I guess with timeslices it eventually always will, but I suspect we could
> > have some serious latency spikes.
>
> Yes, the owner getting preempted after acquiring the lock, but before
> setting the owner can give some nasties :-(
>
> I initially did that preempt_disable/enable around the fast path, but I
> agree that slowing down the fast path is unwelcome.
>
> Alternatively we could go back to block on !owner, with the added
> complexity of not breaking out of the spin on lock->owner != owner
> when !lock->owner, so that the premature owner clearing of the unlock
> fast path will not force a schedule right before we get a chance to
> acquire the lock.
>
> Let me do that..

Ok a few observations..

Adding that need_resched() in the outer loop utterly destroys the
performance gain for PREEMPT=y. Voluntary preemption is mostly good, but
somewhat unstable results.

Adding that blocking on !owner utterly destroys everything.

Going to look into where that extra preemption comes from.