Re: [PATCH][RFC]: mutex: adaptive spin

Peter Zijlstra
2009-01-06T12:22:30Z

On Tue, 2009-01-06 at 13:10 +0100, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
> > +++ linux-2.6/kernel/mutex.c
> > @@ -46,6 +46,7 @@ __mutex_init(struct mutex *lock, const c
> > atomic_set(&lock->count, 1);
> > spin_lock_init(&lock->wait_lock);
> > INIT_LIST_HEAD(&lock->wait_list);
> > + lock->owner = NULL;
> >
> > debug_mutex_init(lock, name, key);
> > }
> > @@ -120,6 +121,28 @@ void __sched mutex_unlock(struct mutex *
> >
> > EXPORT_SYMBOL(mutex_unlock);
> >
> > +#ifdef CONFIG_SMP
> > +static int adaptive_wait(struct mutex_waiter *waiter,
> > + struct task_struct *owner, long state)
> > +{
> > + for (;;) {
> > + if (signal_pending_state(state, waiter->task))
> > + return 0;
> > + if (waiter->lock->owner != owner)
> > + return 0;
> > + if (!task_is_current(owner))
> > + return 1;
> > + cpu_relax();
> > + }
> > +}
> > +#else
>
> Linus, what do you think about this particular approach of spin-mutexes?
> It's not the typical spin-mutex i think.
>
> The thing i like most about Peter's patch (compared to most other adaptive
> spinning approaches i've seen, which all sucked as they included various
> ugly heuristics complicating the whole thing) is that it solves the "how
> long should we spin" question elegantly: we spin until the owner runs on a
> CPU.

s/until/as long as/

> So on shortly held locks we degenerate to spinlock behavior, and only
> long-held blocking locks [with little CPU time spent while holding the
> lock - say we wait for IO] we degenerate to classic mutex behavior.
>
> There's no time or spin-rate based heuristics in this at all (i.e. these
> mutexes are not 'adaptive' at all!), and it degenerates to our primary and
> well-known locking behavior in the important boundary situations.

Well, it adapts to the situation, choosing between spinning vs blocking.
But what's in a name ;-)

> A couple of other properties i like about it:
>
> - A spinlock user can be changed to a mutex with no runtime impact. (no
> increase in scheduling) This might enable us to convert/standardize
> some of the uglier locking constructs within ext2/3/4?

I think a lot of stuff there is bit (spin) locks.

> It's hard to tell how it would impact inbetween workloads - i guess it
> needs to be measured on a couple of workloads.

Matthew volunteered to run something IIRC.