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    SubjectRe: Inquiry: Should we remove "isolcpus= kernel boot option? (may have realtime uses)
    Mark Hounschell wrote:
    > IMHO,
    >
    > What is an abonination, is that cpusets are equired for this type of
    > isolation to begin with, even on a 2 processor machine.
    >
    > I would like the option to stay and be extended like Max originally
    > proposed. If cpusets/hotplug are configured isolation would be obtained
    > using them. If not then isolcpus could be used to get the same isolation.
    >
    > From a user land point of view, I just want an easy way to fully isolate
    > a particular cpu. Even a new syscall or extension to sched_setaffinity
    > would make me happy. Cpusets and hotplug don't.
    >
    > Again this is just MHO.

    Mark, I used to be the same way and I'm a convert now. It does seems like an
    overkill for 2cpu machine to have cpusets and cpu hotplug. But both options
    cost around 50KB worth of text and maybe another 10KB of data. That's on the
    x86-64 box. Let's say it's a 100KB. Not a terribly huge overhead.

    Now if you think about it. In order to be able to dynamically isolate a cpu we
    have to do exact same thing that CPU hotplug does. Which is to clear all
    timers, kernel, threads, etc from that CPUs. It does not make sense to
    implement a separate logic for that. You could argue that you do not need
    dynamic isolation but it's too inflexible in general even on 2way machines
    it's waste to not be able to use second cpu for general load even when RT app
    is not running. Given that CPU hotplug is necessary for many things, including
    suspend on multi-cpu machines it's practically guaranteed to be very stable
    and well supported. In other words we have a perfect synergy here :).

    Now, about the cpusets. You do not really have to do anything fancy with them.
    If all you want to do is to disable systemwide load balancing
    mount -tcgroup -o cpuset cpuset /dev/cpuset
    echo 0 > /dev/cpuset/cpuset.sched_load_banace

    That's it. You get _exactly_ the same effect as with isolcpus=. And you can
    change that dynamically, and when you switch to quad- and eight- core machines
    then you'll be to do that with groups of cpus, not just system wide.

    Just to complete the example above. Lets say you want to isolate cpu2
    (assuming that cpusets are already mounted).

    # Bring cpu2 offline
    echo 0 > /sys/devices/system/cpu/cpu2/online

    # Disable system wide load balancing
    echo 0 > /dev/cpuset/cpuset.sched_load_banace

    # Bring cpu2 online
    echo 1 > /sys/devices/system/cpu/cpu2/online

    Now if you want to un-isolate cpu2 you do

    # Disable system wide load balancing
    echo 1 > /dev/cpuset/cpuset.sched_load_banace

    Of course this is not a complete isolation. There are also irqs (see my
    "default irq affinity" patch), workqueues and the stop machine. I'm working on
    those too and will release .25 base cpuisol tree when I'm done.

    Max






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