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    SubjectRe: [PATCH RFC V6 0/11] Paravirtualized ticketlocks
    On Fri, 30 Mar 2012, H. Peter Anvin wrote:

    > What is the current status of this patchset? I haven't looked at it too
    > closely because I have been focused on 3.4 up until now...

    The real question is whether these heuristics are the correct approach
    or not.

    If I look at it from the non virtualized kernel side then this is ass
    backwards. We know already that we are holding a spinlock which might
    cause other (v)cpus going into eternal spin. The non virtualized
    kernel solves this by disabling preemption and therefor getting out of
    the critical section as fast as possible,

    The virtualization problem reminds me a lot of the problem which RT
    kernels are observing where non raw spinlocks are turned into
    "sleeping spinlocks" and therefor can cause throughput issues for non
    RT workloads.

    Though the virtualized situation is even worse. Any preempted guest
    section which holds a spinlock is prone to cause unbound delays.

    The paravirt ticketlock solution can only mitigate the problem, but
    not solve it. With massive overcommit there is always a way to trigger
    worst case scenarious unless you are educating the scheduler to cope
    with that.

    So if we need to fiddle with the scheduler and frankly that's the only
    way to get a real gain (the numbers, which are achieved by this
    patches, are not that impressive) then the question arises whether we
    should turn the whole thing around.

    I know that Peter is going to go berserk on me, but if we are running
    a paravirt guest then it's simple to provide a mechanism which allows
    the host (aka hypervisor) to check that in the guest just by looking
    at some global state.

    So if a guest exits due to an external event it's easy to inspect the
    state of that guest and avoid to schedule away when it was interrupted
    in a spinlock held section. That guest/host shared state needs to be
    modified to indicate the guest to invoke an exit when the last nested
    lock has been released.

    Of course this needs to be time bound, so a rogue guest cannot
    monopolize the cpu forever, but that's the least to worry about
    problem simply because a guest which does not get out of a spinlocked
    region within a certain amount of time is borked and elegible to
    killing anyway.

    Thoughts ?



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