Re: Q: select_fallback_rq() && cpuset_lock()

[Oleg Nesterov <oleg@redhat.com>]

On 03/11, Peter Zijlstra wrote:
>
> On Thu, 2010-03-11 at 15:52 +0100, Oleg Nesterov wrote:
>
> > Btw, select_fallback_rq() takes
> > rcu_read_lock around cpuset_cpus_allowed_locked(). Why? I must have
> > missed something, but afaics this buys nothing.
>
> for task_cs() iirc.

it should be stable under task_lock()... Never mind.

> > How can we fix this later? Perhaps we can change
> > cpuset_track_online_cpus(CPU_DEAD) to scan all affected cpusets and
> > fixup the tasks with the wrong ->cpus_allowed == cpu_possible_mask.
>
> Problem is, we can't really fix up tasks, wakeup must be able to find a
> suitable cpu.

Yes sure. I meant, wakeup()->select_fallback_rq() sets cpus_allowed =
cpu_possible_map as we discussed. Then cpuset_track_online_cpus(CPU_DEAD)
fixes the affected tasks.

> > At first glance this should work in try_to_wake_up(p) case, we can't
> > race with cpuset_change_cpumask()/etc because of TASK_WAKING logic.
>
> Well, cs->cpus_possible can still go funny on us.

What do you mean? Afaics, cpusets always uses set_cpus_allowed() to
change task->cpus_allowed.

> > But I am not sure how can we fix move_task_off_dead_cpu(). I think
> > __migrate_task_irq() itself is fine, but if select_fallback_rq() is
> > called from move_task_off_dead_cpu() nothing protects ->cpus_allowed.
>
> It has that retry loop in case the migration fails, right?
>
> > We can race with cpusets, or even with the plain set_cpus_allowed().
> > Probably nothing really bad can happen, if the resulting cpumask
> > doesn't have online cpus due to the racing memcpys, we should retry
> > after __migrate_task_irq() fails. Or we can take cpu_rq(cpu)-lock
> > around cpumask_copy(p->cpus_allowed, cpu_possible_mask).
>
> It does the retry thing.

Yes, I mentioned retry logic too. But it can't always help, even without
cpusets.

Suppose a task T is bound to the dead CPU, and move_task_off_dead_cpu()
races with set_cpus_allowed(new_mask). I think it is fine if T gets
either new_mask or cpu_possible_map in ->cpus_allowed. But, it can get
a "random" mix if 2 memcpy() run in parallel. And it is possible that
__migrate_task_irq() will not fail if dest_cpu falls into resulting mask.

> > @@ -2289,10 +2289,9 @@ static int select_fallback_rq(int cpu, s
> >
> >  	/* No more Mr. Nice Guy. */
> >  	if (dest_cpu >= nr_cpu_ids) {
> > -		rcu_read_lock();
> > -		cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
> > -		rcu_read_unlock();
> > -		dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
> > +		// XXX: take cpu_rq(cpu)->lock ???
> > +		cpumask_copy(&p->cpus_allowed, cpu_possible_mask);
> > +		dest_cpu = cpumask_any(cpu_active_mask);
>
>
> Right, this seems safe.

OK, I'll try to read this code a bit more and then send this patch.

Oleg.