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    SubjectRe: [PATCH] sched: fix evaluation of skip_for_load in move_tasks()
    On Tue, Apr 25, 2006 at 01:08:00PM +1000, Peter Williams wrote:
    > Problem:
    >
    > In the patch
    > sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch
    > I got a the sense of a boolean expression wrong when assigning a value
    > to skip_for_load. The expression should have been negated before being
    > assigned.
    >
    > Additionally, busiest_best_prio_seen is being set when tasks are moved
    > instead of when they are skipped which will cause problems when the
    > current task does not have prio=busiest_best_prio.
    >
    > Solution:
    >
    > Negate the expression and apply de Marcos rule to simplify it and move
    > the setting of busiest_best_prio_seen.
    >
    > This patch is on top of
    > sched-avoid-unnecessarily-moving-highest-priority-task-move_tasks.patch
    >
    > Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
    >
    > --
    > Peter Williams pwil3058@bigpond.net.au
    >
    > "Learning, n. The kind of ignorance distinguishing the studious."
    > -- Ambrose Bierce

    > Index: MM-2.6.17-rc1-mm3/kernel/sched.c
    > ===================================================================
    > --- MM-2.6.17-rc1-mm3.orig/kernel/sched.c 2006-04-25 12:53:39.000000000 +1000
    > +++ MM-2.6.17-rc1-mm3/kernel/sched.c 2006-04-25 12:56:14.000000000 +1000
    > @@ -2059,7 +2059,10 @@ static int move_tasks(runqueue_t *this_r
    > busiest_best_prio = rq_best_prio(busiest);
    > /*
    > * Enable handling of the case where there is more than one task
    > - * with the best priority.
    > + * with the best priority. If the current running task is one
    > + * of those with prio==busiest_best_prio we know it won't be moved
    > + * and therefore it's safe to override the skip (based on load) of
    > + * any task we find with that prio.
    > */
    > busiest_best_prio_seen = busiest_best_prio == busiest->curr->prio;
    >
    > @@ -2108,9 +2111,10 @@ skip_queue:
    > */
    > skip_for_load = tmp->load_weight > rem_load_move;
    > if (skip_for_load && idx < this_best_prio)
    > - skip_for_load = busiest_best_prio_seen || idx != busiest_best_prio;
    > + skip_for_load = !busiest_best_prio_seen && idx == busiest_best_prio;

    I think we need to change this to
    if (skip_for_load && idx < this_best_prio && idx == busiest_best_prio)
    skip_for_load = !busiest_best_prio_seen;

    Otherwise we will reset skip_for_load to '0' even for the tasks whose prio is
    less than this_best_prio but not equal to busiest_best_prio.

    > if (skip_for_load ||
    > !can_migrate_task(tmp, busiest, this_cpu, sd, idle, &pinned)) {
    > + busiest_best_prio_seen |= idx == busiest_best_prio;
    > if (curr != head)
    > goto skip_queue;
    > idx++;
    > @@ -2133,8 +2137,6 @@ skip_queue:
    > if (pulled < max_nr_move && rem_load_move > 0) {
    > if (idx < this_best_prio)
    > this_best_prio = idx;
    > - if (idx == busiest_best_prio)
    > - busiest_best_prio_seen = 1;
    > if (curr != head)
    > goto skip_queue;
    > idx++;
    -
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