Messages in this thread | | | Subject | Re: Q: select_fallback_rq() && cpuset_lock() | From | Peter Zijlstra <> | Date | Thu, 11 Mar 2010 16:35:59 +0100 |
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On Thu, 2010-03-11 at 15:52 +0100, Oleg Nesterov wrote: > On 03/10, Oleg Nesterov wrote: > > > > On 03/10, Peter Zijlstra wrote: > > > > > > Right, so if you refresh these patches, I'll feed them to mingo and they > > > should eventually end up in -linus, how's that? :-) > > > > Great. Will redo/resend tomorrow ;) > > That was a great plan, but it doesn't work. > > With the recent changes we have even more problems with > cpuset_cpus_allowed_locked(). Not only it misses cpuset_lock() (which > doesn't work anyway and must die imho), it is very wrong to even call > this function from try_to_wakeup() - this can deadlock. > > Because task_cs() is protected by task_lock() which is not irq-safe, > and cpuset_cpus_allowed_locked() takes this lock.
You're right, and lockdep doesn't normally warn about that because nobody really hits this path :/
> We need more changes in cpuset.c. 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.
> From the previous email: > > On 03/10, Peter Zijlstra wrote: > > > > On Wed, 2010-03-10 at 18:30 +0100, Oleg Nesterov wrote: > > > On 03/10, Peter Zijlstra wrote: > > > > > > > > I guess the quick fix is to really bail and always use cpu_online_mask > > > > in select_fallback_rq(). > > > > > > Yes, but this breaks cpusets. > > > > Arguably, so, but you can also argue that binding a task to a cpu and > > then unplugging that cpu without first fixing up the affinity is a 'you > > get to keep both pieces' situation. > > Well yes, but still it was supposed the kernel should handle this case > correctly, the task shouldn't escape its cpuset. > > However, currently I don't see another option. I think we should fix the > possible deadlocks and kill cpuset_lock/cpuset_cpus_allowed_locked first, > then try to fix cpusets. > > See the trivial (uncompiled) patch below. It just states a fact > cpuset_cpus_allowed() logic is broken. > > 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.
> 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.
> 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.
> sched_exec() seems fine, the task is current and running, > "No more Mr. Nice Guy." case is not possible. > > What do you think? > > Btw, I think there is a small bug in set_cpus_allowed_ptr(), > wake_up_process(rq->migration_thread) can hit NULL, we should do > wake_up_process(mt).
Agreed.
> @@ -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.
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