Messages in this thread | | | Date | Fri, 24 Jun 2016 15:09:32 +0200 | From | Peter Zijlstra <> | Subject | Re: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and SPECjvm2008 benchmarks against 4.6 kernel |
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On Fri, Jun 24, 2016 at 02:44:07PM +0200, Vincent Guittot wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 22d64b3f5876..d4f6fb2f3057 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -2484,7 +2484,7 @@ static inline long calc_tg_weight(struct task_group *tg, struct cfs_rq *cfs_rq) > > */ > > tg_weight = atomic_long_read(&tg->load_avg); > > tg_weight -= cfs_rq->tg_load_avg_contrib; > > - tg_weight += cfs_rq->load.weight; > > + tg_weight += cfs_rq->avg.load_avg; > > IIUC, you are reverting > commit fde7d22e01aa (sched/fair: Fix overly small weight for > interactive group entities)
Ah!, I hadn't yet done a git-blame on this. Right you are, we should have put a comment there.
So the problem here is that since commit:
2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
load.weight and avg.load_avg are in different metrics. Which completely wrecked things.
The obvious alternative is using:
scale_load_down(cfs_rq->load.weight);
Let me go run that through the benchmark.
> I have one question regarding the use of cfs_rq->avg.load_avg > cfs_rq->tg_load_avg_contrib is the sampling of cfs_rq->avg.load_avg so > I'm curious to understand why you use cfs_rq->avg.load_avg instead of > keeping cfs_rq->tg_load_avg_contrib. Do you think that the sampling is > not accurate enough to prevent any significant difference between both > when we use tg->load_avg ?
I'm not entirely sure I understand your question; is it to the existence of calc_tg_weight()? That is, why use calc_tg_weight() and not use tg->load_avg as is?
It seemed like a simple and cheap way to increase accuracy, nothing more behind it until the commit you referred to.
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