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    SubjectRe: [PATCH 0/7] RT: (RFC) RT-Overload/Sched enhancements
    On Fri, 2007-10-12 at 12:29 +0200, Peter Zijlstra wrote:

    > I'm wondering why we need the cpu prio management stuff.

    I know we covered most of this on IRC, but let me recap so everyone can
    follow the thread:

    1) The cpupri alg is just one search alg vs the other. I think we are
    all in agreement that it is not clear if one has an advantage over the
    other without some empirical data. So that aspect still remains to be
    investigated. For now, either can be interchanged as the main
    functionality is what uses the search, not the search alg itself.

    2) The patch series makes some innovations above the current state of
    the push-rt patch, which I will try to summarize here for consideration:

    A) CPU Priority should be updated due to PI changes as well as

    B) The search algorithm in the cpupri alg employs a priority amongst
    eligible CPUs: last-run (for cache-affinity), this_cpu (for
    lower-overhead preemption), and finally any other cpu in the
    cpus_allowed. It would be ideal to see the other alg provide similar

    C) The search and pusher functions are separated. Search is useful in
    circumstances outside the push_rt_task functions (see (D))

    D) The primary patch addresses one case where we need to redistribute
    (high-pri preemption). There are 3 in total. The series adds support
    for a second case (low-pri RT wakeup).

    E) We push until equilibrium instead of just a single task.

    3) Having a distinct cpu-priority layer (regardless of search-arg) will
    have (IMO) interesting potential going forward. For instance, we could
    have an optional notifier that gets kicked whenever we change
    priority-class. This would allow for some interesting RT related
    enhancements by allowing system-level components to register for
    priority changes. For example: APIC TPR, or KVM hypercalls (for RT
    guests, on an RT host). This is more theoretical and half-baked at this
    point, but it was something I have been kicking around.

    That's all I can think of for now.


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