Messages in this thread | | | Subject | Re: Q: select_fallback_rq() && cpuset_lock() | From | Peter Zijlstra <> | Date | Thu, 11 Mar 2010 17:29:13 +0100 |
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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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