On 10/11, Peter Zijlstra wrote: > > On Fri, Oct 11, 2013 at 08:25:07PM +0200, Oleg Nesterov wrote: > > On 10/11, Peter Zijlstra wrote: > > > > > > As a penance I'll start by removing all get_online_cpus() usage from the > > > scheduler. > > > > I only looked at the change in setaffinity, > > > > > @@ -3706,7 +3707,6 @@ long sched_setaffinity(pid_t pid, const struct > > > cpumask *in_mask) > > > struct task_struct *p; > > > int retval; > > > > > > - get_online_cpus(); > > > rcu_read_lock(); > > > > Hmm. In theory task_rq_lock() doesn't imply rcu-lock, so > > set_cpus_allowed_ptr() can miss the change in cpu_active_mask. But this > > is probably fine, CPU_DYING does __migrate_task(). > > I'm fine with always doing sync_sched(); sync_rcu(); if that makes you > feel better.
No, I was just curious. iow, I am asking, not arguing. > But I thought that assuming that !PREEMPT sync_rcu() would > imply sync_sched() was ok. I think the comment there even says as much. > > And task_rq_lock() will very much disable preemption; and thus we get > what we want, right? it even disables irqs, so this should always imply rcu_read_lock() with any implementation, I guess. However I was told we should not rely on this, and say posix_timer_event() does rcu_read_lock() even it is always called under spin_lock_irq(). But what I actually tried to say, it seems that this particular change looks fine even if cpu_down() doesn't do sync_sched/rcu at all, because we can rely on __migrate_task(). IOW, if we race with DOWN_PREPARE and miss set_cpu_active(false) we can pretend that this CPU goes down later. > In any case; the goal was to make either RCU or preempt-disable > sufficient. > > > However. This means that sched_setaffinity() can fail if it races with > > the failing cpu_down() (say, __cpu_notify(CPU_DOWN_PREPARE) fails). > > Probably we do not really care, just this looks a bit confusing. > > Couldn't be bothered; failing hotplug will have side-effects any which > way. OK. > > > @@ -3827,12 +3825,11 @@ long sched_getaffinity(pid_t pid, struct cpumask > > > *mask) > > > goto out_unlock; > > > > > > raw_spin_lock_irqsave(&p->pi_lock, flags); > > > - cpumask_and(mask, &p->cpus_allowed, cpu_online_mask); > > > + cpumask_and(mask, &p->cpus_allowed, cpu_active_mask); > > > > But I am just curious, is this change is strictly needed? > > No; we could do without. It really doesn't matter much if anything. I > only did it because sched_setaffinity()->set_cpus_allowed_ptr() checks > against active, not online. And had a sudden urge to make get/set > symmetric -- totally pointless otherwise. OK, thanks, I was just curious. In fact I do not even understand why getaffinity() doesn't simply return ->cpus_allowed, but this is off-topic. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
