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    SubjectRe: [patch] fix SMT scheduler latency bug
    Date
    On Thu, 23 Jun 2005 02:04, Ingo Molnar wrote:
    > * Con Kolivas <kernel@kolivas.org> wrote:
    > > > [...]
    > > > if (((smt_curr->time_slice * (100 -
    > > > sd->per_cpu_gain) / 100) > task_timeslice(p)))
    > > > ret = 1;
    > > >
    > > > note that in contrast to the comment above, we dont actually do the
    > > > check based on static priority, we do the check based on timeslices.
    > > > But timeslices go up and down, and even highprio tasks can randomly
    > > > have very low timeslices (just before their next refill) and can thus
    > > > be judged as 'lowprio' by the above piece of code.
    > >
    > > I don't see it like that. task_timeslice(p) will always return the
    > > same value based purely on static priority and smt_curr->time_slice
    > > cannot ever be larger than task_timeslice(p) unless there is a
    > > significant enough 'nice' difference. [...]
    >
    > task_timeslice(p) is indeed constant over time, but smt_curr->time_slice
    > is not. So this condition opens up the possibility of a lower prio
    > thread accumulating a larger ->time_slice and thus reversing the
    > priority equation.

    I'm not clear on how the value of ->time_slice can ever grow to larger than
    task_timeslice(p). It starts at task_timeslice(p) and decrements till it gets
    to 0 when it refills again.

    I only recall three times that the value of p->time_slice is increased:
    New timeslice where it is refilled with task_timeslice(p)
    At fork() where if it has one tick left it is increased back to one tick
    Reaping a child where it collects its child's time_slice up to a maximum of
    task_timeslice(p).

    I must be missing something but I can't see how p->time_slice can ever be
    larger than task_timeslice(p).

    Please correct me :|

    Cheers,
    Con
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