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

Steven Rostedt
2009-01-08T15:09:30Z

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 ;-)

> + 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?

-- 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
> +
> #ifdef CONFIG_PREEMPT
> /*
> * this is the entry point to schedule() from in-kernel preemption
> Index: linux-2.6/include/linux/sched.h
> ===================================================================
> --- linux-2.6.orig/include/linux/sched.h
> +++ linux-2.6/include/linux/sched.h
> @@ -330,6 +330,7 @@ extern signed long schedule_timeout_inte
> extern signed long schedule_timeout_killable(signed long timeout);
> extern signed long schedule_timeout_uninterruptible(signed long timeout);
> asmlinkage void schedule(void);
> +extern int spin_on_owner(struct mutex *lock, struct thread_info *owner);
>
> struct nsproxy;
> struct user_namespace;
> Index: linux-2.6/kernel/sched_features.h
> ===================================================================
> --- linux-2.6.orig/kernel/sched_features.h
> +++ linux-2.6/kernel/sched_features.h
> @@ -13,3 +13,4 @@ SCHED_FEAT(LB_WAKEUP_UPDATE, 1)
> SCHED_FEAT(ASYM_EFF_LOAD, 1)
> SCHED_FEAT(WAKEUP_OVERLAP, 0)
> SCHED_FEAT(LAST_BUDDY, 1)
> +SCHED_FEAT(OWNER_SPIN, 1)
>
>
>