Messages in this thread | | | Date | Tue, 30 Jan 2007 08:33:40 +0100 | From | Ingo Molnar <> | Subject | Re: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug |
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* Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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.
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