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    SubjectRe: scheduler scalability - cgroups, cpusets and load-balancing

    On Tue, 2008-01-29 at 05:13 -0600, Paul Jackson wrote:
    > Peter wrote:
    > > Thanks for the link. Yes I think your last suggestion of creating
    > > rt-domains ( ) is a good one.
    > We now have a per-cpuset Boolean flag file called 'sched_load_balance'.

    SD_LOAD_BALANCE, right?

    > In the default case, this flag is set on, and the kernel does its
    > usual load balancing across all CPUs in that cpuset. This means, under
    > the covers, that there exists some sched domain such that all CPUs in
    > that cpuset are in that same sched domain. That sched domain might
    > contain additional CPUs from outside that cpuset as well. Indeed,
    > in the default vanilla configuration, that sched domain contains all
    > CPUs in the system.
    > If we turn the sched_load_balance flag off for some cpuset, we are
    > telling the kernel it's ok not to load balance on the CPUs in that
    > cpuset (unless those CPUs are in some other cpuset that needed load
    > balancing anyway.)
    > This 'sched_load_balance' flag is, thus far, "the" cpuset hook
    > supporting realtime. One can use it to configure a system so that
    > the kernel does not do normal load balancing on select CPUs, such
    > as those CPUs dedicated to realtime use.

    Ah, here I disagree, it is possible to do (hard) realtime scheduling
    over multiple cpus, the only draw back is that it requires a very strong
    load-balancer, making it unsuitable for large number of cpus.

    ( of course, having a strong rt load balancer on a large cpuset doesn't
    harm, as long as there are no rt tasks to balance )

    So if we have a system like so:

    / | \
    B1 B2 B3
    / \
    C1 C2

    A comprises of cpus 0-127, !SD_LOAD_BALANCE

    B1 comprises of cpus 0-63, !SD_LOAD_BALANCE
    B2 comprises of cpus 64-119
    B3 120-127

    C1 0-3
    C2 5-63

    We end up with 4 disjoint load-balanced sets.

    I would then attach the rt balance information to: C1, C2, B2, B3.

    If, for example, B1 would be load-balanced, we'd only have 3 disjoint
    sets left: B1, B2 and B3, and the rt balance data would be there.

    > It sounds like Peter is reminding us that we really have three choices
    > for a handling a given CPU's load balancing:
    > 1) normal kernel scheduler load balancing,
    > 2) RT load balancing, or
    > 3) no load balancing whatsoever.
    > If that's the case (if we really need choice 3) then a single Boolean
    > flag, such as sched_load_balance, is not sufficient to select from
    > the three choices, and it might make sense to add a second per-cpuset
    > Boolean flag, say "sched_rt_balance", default off, which if turned on,
    > enabled choice 2.
    > If that's not the case (we only need choices 1 and 2) then -logically-
    > we could overload the meaning of the current sched_load_balance,
    > to mean, if turned off, not only to stop doing normal balancing, but
    > to further mean that we should commence RT balancing. However bits
    > aren't -that- precious here, and this sounds unnecessarily confusing.
    > So ... would a new per-cpuset Boolean flag such as sched_rt_balance be
    > appropriate and sufficient to mark those cpusets whose set of CPUs
    > required RT balancing?

    So, I don't think we need that, I think we can do with the single flag,
    we just need to find these disjoint sets and stick our rt-domain there.

     \ /
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