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    SubjectRe: Fw: Re: [mm PATCH 4/6] RCU: (now) CPU hotplug
    On Tue, Jan 30, 2007 at 08:33:40AM +0100, Ingo Molnar wrote:
    >
    > * 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 :-)

    Well, it turns out that maybe it is in fact untrue. :-/

    I need to look at all uses of PF_NOFREEZE -- as I understand the
    code, processes marked PF_NOFREEZE will continue running, potentially
    interfering with the hotplug operation. :-(

    I will pass my findings on to this list.

    > 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 ;-)

    Hey, just ending the debates on how to do CPU-hotplug locking would be
    worth something! ;-)

    > 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.

    :-(

    But there can't be -that- many elements in that list... But agreed,
    separate issue.

    Thanx, Paul
    -
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