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

Steven Rostedt
2009-01-08T15:28:05Z

On Thu, 8 Jan 2009, Peter Zijlstra wrote:

> On Thu, 2009-01-08 at 10:09 -0500, Steven Rostedt wrote:
> > On Thu, 8 Jan 2009, Peter Zijlstra wrote:
> > > Index: linux-2.6/kernel/sched.c
> > > ===================================================================
> > > --- linux-2.6.orig/kernel/sched.c
> > > +++ linux-2.6/kernel/sched.c
> > > @@ -4672,6 +4672,72 @@ need_resched_nonpreemptible:
> > > }
> > > EXPORT_SYMBOL(schedule);
> > >
> > > +#ifdef CONFIG_SMP
> > > +/*
> > > + * Look out! "owner" is an entirely speculative pointer
> > > + * access and not reliable.
> > > + */
> > > +int spin_on_owner(struct mutex *lock, struct thread_info *owner)
> > > +{
> > > + unsigned int cpu;
> > > + struct rq *rq;
> > > + int ret = 1;
> > > +
> > > + if (unlikely(!sched_feat(OWNER_SPIN)))
> >
> > I would remove the "unlikely", if someone turns OWNER_SPIN off, then you
> > have the wrong decision being made. Choices by users should never be in a
> > "likely" or "unlikely" annotation. It's discrimination ;-)
>
> in the unlikely case we schedule(), that seems expensive enough to want
> to make the spin case ever so slightly faster.

OK, that makes sense, but I would comment that. Otherwise, it just looks
like another misuse of the unlikely annotation.

>
> > > + return 0;
> > > +
> > > + preempt_disable();
> > > +#ifdef CONFIG_DEBUG_PAGEALLOC
> > > + /*
> > > + * Need to access the cpu field knowing that
> > > + * DEBUG_PAGEALLOC could have unmapped it if
> > > + * the mutex owner just released it and exited.
> > > + */
> > > + if (probe_kernel_address(&owner->cpu, cpu))
> > > + goto out;
> > > +#else
> > > + cpu = owner->cpu;
> > > +#endif
> > > +
> > > + /*
> > > + * Even if the access succeeded (likely case),
> > > + * the cpu field may no longer be valid.
> > > + */
> > > + if (cpu >= nr_cpumask_bits)
> > > + goto out;
> > > +
> > > + /*
> > > + * We need to validate that we can do a
> > > + * get_cpu() and that we have the percpu area.
> > > + */
> > > + if (!cpu_online(cpu))
> > > + goto out;
> >
> > Should we need to do a "get_cpu" or something? Couldn't the CPU disappear
> > between these two calls. Or does it do a stop-machine and the preempt
> > disable will protect us?
>
> Did you miss the preempt_disable() a bit up?

No, let me rephrase it better. Does the preempt_disable protect against
another CPU from going off line? Does taking a CPU off line do a
stop_machine?

-- Steve

>
> > > +
> > > + rq = cpu_rq(cpu);
> > > +
> > > + for (;;) {
> > > + if (lock->owner != owner)
> > > + break;
> > > +
> > > + /*
> > > + * Is that owner really running on that cpu?
> > > + */
> > > + if (task_thread_info(rq->curr) != owner)
> > > + break;
> > > +
> > > + if (need_resched()) {
> > > + ret = 0;
> > > + break;
> > > + }
> > > +
> > > + cpu_relax();
> > > + }
> > > +out:
> > > + preempt_enable_no_resched();
> > > + return ret;
> > > +}
> > > +#endif
>
>
>