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    Subject[PATCH 5/8] sched: virtual time buddy preemption
    Since we moved wakeup preemption back to virtual time, it makes sense to move
    the buddy stuff back as well. The purpose of the buddy scheduling is to allow
    a quickly scheduling pair of tasks to run away from the group as far as a
    regular busy task would be allowed under wakeup preemption.

    This has the advantage that the pair can ping-pong for a while, enjoying
    cache-hotness. Without buddy scheduling other tasks would interleave destroying
    the cache.

    Also, it saves a word in cfs_rq.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    kernel/sched.c | 1 -
    kernel/sched_fair.c | 12 +++++-------
    2 files changed, 5 insertions(+), 8 deletions(-)

    Index: linux-2.6/kernel/sched.c
    ===================================================================
    --- linux-2.6.orig/kernel/sched.c
    +++ linux-2.6/kernel/sched.c
    @@ -385,7 +385,6 @@ struct cfs_rq {

    u64 exec_clock;
    u64 min_vruntime;
    - u64 pair_start;

    struct rb_root tasks_timeline;
    struct rb_node *rb_leftmost;
    Index: linux-2.6/kernel/sched_fair.c
    ===================================================================
    --- linux-2.6.orig/kernel/sched_fair.c
    +++ linux-2.6/kernel/sched_fair.c
    @@ -790,16 +790,14 @@ set_next_entity(struct cfs_rq *cfs_rq, s
    se->prev_sum_exec_runtime = se->sum_exec_runtime;
    }

    +static int
    +wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
    +
    static struct sched_entity *
    pick_next(struct cfs_rq *cfs_rq, struct sched_entity *se)
    {
    - struct rq *rq = rq_of(cfs_rq);
    - u64 pair_slice = rq->clock - cfs_rq->pair_start;
    -
    - if (!cfs_rq->next || pair_slice > sysctl_sched_min_granularity) {
    - cfs_rq->pair_start = rq->clock;
    + if (!cfs_rq->next || wakeup_preempt_entity(cfs_rq->next, se) == 1)
    return se;
    - }

    return cfs_rq->next;
    }
    --



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