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SubjectRe: [PATCH -v7][RFC]: mutex: implement adaptive spinning
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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.

> > + 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?

> > +
> > + 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



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