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

[Peter Zijlstra]

On Thu, 2010-03-11 at 17:19 +0100, Oleg Nesterov wrote:

> > > 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.

Ah, have that re-validate the p->cpus_allowed for all cpuset tasks, ok
that might work.

> > > 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.

Confusion^2 ;-), I failed to grasp your fixup idea and got confused,
which confused you.

> > > 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.

Ah indeed. One would almost construct a cpumask_assign that uses RCU
atomic pointer assignment for all this stupid cpumask juggling :/

> > > @@ -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.

Thanks!

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