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    SubjectRe: RFC [patch] sched: strengthen LAST_BUDDY and minimize buddy induced latencies V3
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    On Sat, 2009-10-17 at 12:24 +0200, Mike Galbraith wrote:
    > sched: strengthen LAST_BUDDY and minimize buddy induced latencies.
    >
    > This patch restores the effectiveness of LAST_BUDDY in preventing pgsql+oltp
    > from collapsing due to wakeup preemption. It also minimizes buddy induced
    > latencies. x264 testcase spawns new worker threads at a high rate, and was
    > being affected badly by NEXT_BUDDY. It turned out that CACHE_HOT_BUDDY was
    > thwarting idle balancing. This patch ensures that the load can disperse,
    > and that buddies can't make any task excessively late.

    > Index: linux-2.6/kernel/sched.c
    > ===================================================================
    > --- linux-2.6.orig/kernel/sched.c
    > +++ linux-2.6/kernel/sched.c
    > @@ -2007,8 +2007,12 @@ task_hot(struct task_struct *p, u64 now,
    >
    > /*
    > * Buddy candidates are cache hot:
    > + *
    > + * Do not honor buddies if there may be nothing else to
    > + * prevent us from becoming idle.
    > */
    > if (sched_feat(CACHE_HOT_BUDDY) &&
    > + task_rq(p)->nr_running >= sched_nr_latency &&
    > (&p->se == cfs_rq_of(&p->se)->next ||
    > &p->se == cfs_rq_of(&p->se)->last))
    > return 1;

    I'm not sure about this. The sched_nr_latency seems arbitrary, 1 seems
    like a more natural boundary.

    Also, one thing that arjan found was that we don't need to consider
    buddies cache hot if we're migrating them within a cache domain. So we
    need to add a SD_flag and sched_domain to properly represent the cache
    hierarchy.



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