Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug

[Ingo Molnar]

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> > in fact (new) kprobes uses the freezer, and it's far more 
> > performance sensitive than the handling of CPU hotplug events.
> 
> Outside of realtime workloads, I agree that performance should not be 
> a problem.  And I don't know of any reason why realtime systems need 
> to be able to do hotplug CPU.  Yet, anyway.  ;-)

even for -rt it's not really an issue: the hotplug locks are so 
all-encompassing and so unbound at the moment that there's no realistic 
expectation for them to ever become deterministic. So we might as well 
make them encompass "everything" - without any noticeable effect.

> So the thought is to make _cpu_down() and _cpu_up() do something like 
> the following (untested, probably does not even compile), perhaps with 
> suitable adjustments elsewhere as well?
> 
> 	static int _cpu_down(unsigned int cpu)
> 	{
> 		int err;
> 		struct task_struct *p;
> 		cpumask_t old_allowed, tmp;
> 
> 		if (num_online_cpus() == 1)
> 			return -EBUSY;
> 
> 		if (!cpu_online(cpu))
> 			return -EINVAL;
> 
> 		if (freeze_processes()) {
> 			err = -EBUSY;
> 			goto out_freeze_notify_failed;
> 		}
> 		err = raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
> 							(void *)(long)cpu);

yeah. This all looks so nice that i almost cannot believe it's true :-) 
This would allow us to rip out all the cpu-hotplug locking: wow! If only
someone would volunteer to try to pull this off and then to touch so
many subsystems ;-)

i fully agree that the opposite notifications should be traversed in 
inverse order [but this is an orthogonal improvement]. Too bad the 
notifier list is a single linked list.

	Ingo
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