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    SubjectRe: [RFC] perf_events: ctx_flexible_sched_in() not maximizing PMU utilization
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    On Thu, 2010-05-06 at 16:03 +0200, Stephane Eranian wrote:
    > Hi,
    >
    > Looking at ctx_flexible_sched_in(), the logic is that if group_sched_in()
    > fails for a HW group, then no other HW group in the list is even tried.
    > I don't understand this restriction. Groups are independent of each other.
    > The failure of one group should not block others from being scheduled,
    > otherwise you under-utilize the PMU.
    >
    > What is the reason for this restriction? Can we lift it somehow?

    Sure, but it will make scheduling much more expensive. The current
    scheme will only ever check the first N events because it stops at the
    first that fails, and since you can max fix N events on the PMU its
    constant time.

    To fix this issue you'd have to basically always iterate all events and
    only stop once the PMU is fully booked, which reduces to an O(n) worst
    case algorithm.

    But yeah, I did think of making the thing an RB-tree and basically
    schedule on service received, that should fix the lop-sided RR we get
    with constrained events.


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