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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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