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

Chris Mason
2009-01-09T15:12:58Z

On Fri, 2009-01-09 at 16:06 +0100, Peter Zijlstra wrote:
> 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.

How about if (!owner && need_resched()) break; instead of the
unconditional need_resched(). That should solve the race that Linus saw
without and hurt PREEMPT less.

>
> Adding that blocking on !owner utterly destroys everything.
>
> Going to look into where that extra preemption comes from.

-chris