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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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