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    SubjectRe: [PATCH v10 5/9] sched/fair: Take into account latency priority at wakeup
    On Fri, Jan 13, 2023 at 03:12:30PM +0100, Vincent Guittot wrote:

    > diff --git a/include/linux/sched.h b/include/linux/sched.h
    > index 6c61bde49152..38decae3e156 100644
    > --- a/include/linux/sched.h
    > +++ b/include/linux/sched.h
    > @@ -568,6 +568,8 @@ struct sched_entity {
    > /* cached value of my_q->h_nr_running */
    > unsigned long runnable_weight;
    > #endif
    > + /* preemption offset in ns */
    > + long latency_offset;

    I wonder about the type here; does it make sense to have it depend on
    the bitness; that is if s32 is big enough on 32bit then surely it is so
    too on 64bit, and if not, then it should be unconditionally s64.


    > +static void set_latency_offset(struct task_struct *p)
    > +{
    > + long weight = sched_latency_to_weight[p->latency_prio];
    > + s64 offset;
    > +
    > + offset = weight * get_sleep_latency(false);
    > + offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
    > + p->se.latency_offset = (long)offset;
    > +}

    > +/*
    > + * latency weight for wakeup preemption
    > + */
    > +const int sched_latency_to_weight[40] = {
    > + /* -20 */ -1024, -973, -922, -870, -819,
    > + /* -15 */ -768, -717, -666, -614, -563,
    > + /* -10 */ -512, -461, -410, -358, -307,
    > + /* -5 */ -256, -205, -154, -102, -51,
    > + /* 0 */ 0, 51, 102, 154, 205,
    > + /* 5 */ 256, 307, 358, 410, 461,
    > + /* 10 */ 512, 563, 614, 666, 717,
    > + /* 15 */ 768, 819, 870, 922, 973,
    > +};

    I'm slightly confused by this table, isn't that simply the linear
    function?

    Isn't all that the same as:

    se->se.latency_offset = get_sleep_latency * nice / (NICE_LATENCY_WIDTH/2);

    ? The reason we have prio_to_weight[] is because it's an exponential,
    which is a bit more cumbersome to calculate, but surely we can do a
    linear function at runtime.


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