Re: [PATCH 1/2] cpuset: fix cpuset_cpus_allowed_fallback() don'tupdate tsk->rt.nr_cpus_allowed

[KOSAKI Motohiro]

Hello,

> > > > (3) schedule() makes fallback and cpuset_cpus_allowed_fallback
> > > >      change task->cpus_allowed
> > >
> > > I'm failing to see how this is happening, surely that kthread isn't
> > > actually running that early?
> >
> > If my understand correctly, current call graph is below.
> >
> > kernel_init()
> >         smp_init();
> >                 cpu_up()
> >                         ... cpu hotplug notification
> >                                 kthread_create()
> >         sched_init_smp();
> >
> >
> > So, cpu hotplug event is happen before sched_init_smp(). The old rule is,
> > all kthreads of using cpu-up notification have to use kthread_bind(). It
> > protected from sched load balance.
> >
> > but, now cpuset_cpus_allowed_fallback() forcedly change kthread's cpumask.
> > Why is this works? the point are two.
> >
> > - rcu_cpu_kthread_should_stop() call set_cpus_allowed_ptr() again
> > periodically.
> >   then, it can reset cpumask if cpuset_cpus_allowed_fallback() change it.
> >   my debug print obseve following cpumask change occur at boot time.
> >      1) kthread_bind: bind cpu1
> >      2) cpuset_cpus_allowed_fallback: bind possible cpu
> >      3) rcu_cpu_kthread_should_stop: rebind cpu1
> > - while tsk->rt.nr_cpus_allowed == 1, sched load balancer never be crash.
>
> Seems rcu_spawn_one_cpu_kthread() call wake_up_process() directly,
> which is under hotplug event CPU_UP_PREPARE. Maybe it should be
> under CPU_ONLINE.

Hmm..

I haven't catch your point. cpu_up() call both CPU_UP_PREPARE and CPU_ONLINE
notification. Thus, CPU_ONLINE still be called before sched_init_smp().

Am I missing something?


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/