Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)

KOSAKI Motohiro
2010-01-13T04:47:55Z

> * KOSAKI Motohiro (kosaki.motohiro@jp.fujitsu.com) wrote:
> > Hi
> >
> > Interesting patch :)
> >
> > I have few comments.
> >
> > > Index: linux-2.6-lttng/kernel/sched.c
> > > ===================================================================
> > > --- linux-2.6-lttng.orig/kernel/sched.c 2010-01-12 10:25:47.000000000 -0500
> > > +++ linux-2.6-lttng/kernel/sched.c 2010-01-12 14:33:20.000000000 -0500
> > > @@ -10822,6 +10822,117 @@ struct cgroup_subsys cpuacct_subsys = {
> > > };
> > > #endif /* CONFIG_CGROUP_CPUACCT */
> > >
> > > +#ifdef CONFIG_SMP
> > > +
> > > +/*
> > > + * Execute a memory barrier on all active threads from the current process
> > > + * on SMP systems. Do not rely on implicit barriers in IPI handler execution,
> > > + * because batched IPI lists are synchronized with spinlocks rather than full
> > > + * memory barriers. This is not the bulk of the overhead anyway, so let's stay
> > > + * on the safe side.
> > > + */
> > > +static void membarrier_ipi(void *unused)
> > > +{
> > > + smp_mb();
> > > +}
> > > +
> > > +/*
> > > + * Handle out-of-mem by sending per-cpu IPIs instead.
> > > + */
> > > +static void membarrier_retry(void)
> > > +{
> > > + struct mm_struct *mm;
> > > + int cpu;
> > > +
> > > + for_each_cpu(cpu, mm_cpumask(current->mm)) {
> > > + spin_lock_irq(&cpu_rq(cpu)->lock);
> > > + mm = cpu_curr(cpu)->mm;
> > > + spin_unlock_irq(&cpu_rq(cpu)->lock);
> > > + if (current->mm == mm)
> > > + smp_call_function_single(cpu, membarrier_ipi, NULL, 1);
> > > + }
> > > +}
> > > +
> > > +#endif /* #ifdef CONFIG_SMP */
> > > +
> > > +/*
> > > + * sys_membarrier - issue memory barrier on current process running threads
> > > + * @expedited: (0) Lowest overhead. Few milliseconds latency.
> > > + * (1) Few microseconds latency.
> >
> > Why do we need both expedited and non-expedited mode? at least, this documentation
> > is bad. it suggest "you have to use non-expedited mode always!".
>
> Right. Maybe I should rather write:
>
> + * @expedited: (0) Low overhead, but slow execution (few milliseconds)
> + * (1) Slightly higher overhead, fast execution (few microseconds)
>
> And I could probably go as far as adding a few paragraphs:
>
> Using the non-expedited mode is recommended for applications which can
> afford leaving the caller thread waiting for a few milliseconds. A good
> example would be a thread dedicated to execute RCU callbacks, which
> waits for callbacks to enqueue most of the time anyway.
>
> The expedited mode is recommended whenever the application needs to have
> control returning to the caller thread as quickly as possible. An
> example of such application would be one which uses the same thread to
> perform data structure updates and issue the RCU synchronization.
>
> It is perfectly safe to call both expedited and non-expedited
> sys_membarriers in a process.
>
>
> Does that help ?

Do librcu need both? I bet average programmer don't understand this
explanation. please recall, syscall interface are used by non kernel
developers too. If librcu only use either (0) or (1), I hope remove
another one.

But if librcu really need both, the above explanation is enough good.
I think.

> > > + * Memory barrier on the caller thread _before_ sending first
> > > + * IPI. Matches memory barriers around mm_cpumask modification in
> > > + * switch_mm().
> > > + */
> > > + smp_mb();
> > > + if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL)) {
> > > + membarrier_retry();
> > > + goto unlock;
> > > + }
> >
> > if CONFIG_CPUMASK_OFFSTACK=1, alloc_cpumask_var call kmalloc. FWIW,
> > kmalloc calling seems destory the worth of this patch.
>
> Why ? I'm not sure I understand your point. Even if we call kmalloc to
> allocate the cpumask, this is a constant overhead. The benefit of
> smp_call_function_many() over smp_call_function_single() is that it
> scales better by allowing to broadcast IPIs when the architecture
> supports it. Or maybe I'm missing something ?

It depend on what mean "constant overhead". kmalloc might cause
page reclaim and undeterministic delay. I'm not sure (1) How much
membarrier_retry() slower than smp_call_function_many and (2) Which do
you think important average or worst performance. Only I note I don't
think GFP_KERNEL is constant overhead.

hmm...
Do you intend to GFP_ATOMIC?

> >
> > #ifdef CONFIG_CPUMASK_OFFSTACK
> > membarrier_retry();
> > goto unlock;
> > #endif
> >
> > is better? I'm not sure.
>
> Thanks for the comments !
>
> Mathieu
>