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Date: Tue, 20 Oct 2009 07:03:37 +0200 From: Mike Galbraith <efault@....de> To: Peter Zijlstra <a.p.zijlstra@...llo.nl> Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>, Arjan van de Ven <arjan@...ux.intel.com> Subject: Re: RFC [patch] sched: strengthen LAST_BUDDY and minimize buddy induced latencies V3 On Tue, 2009-10-20 at 06:24 +0200, Peter Zijlstra wrote: > 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. That's what I did first, which of course worked fine. What I'm thinking of doing instead though is to specifically target the only time I see the problem, ie fork/exec load wanting to disperse. I don't really want to see buddies being ripped away from their cache. But as you note below, that can be a good thing iff it lands on a shared cache. In my case, there's a 1 in 3 chance of safe landing. > 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. Yeah, I thought about this too. If there's any overlap time, waking CPU affine is a loser if there's an idle shared cache next door. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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