Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 24 Jun 2016 15:23:37 +0200 | Subject | Re: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and SPECjvm2008 benchmarks against 4.6 kernel |
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On 24 June 2016 at 15:09, Peter Zijlstra <peterz@infradead.org> wrote: > 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.
Yes, looks to be good alternative
> >> 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?
Yes
> > It seemed like a simple and cheap way to increase accuracy, nothing more > behind it until the commit you referred to.
Thanks for the clarification. I thought that the difference should always be smaller than 1/64th of the cfs_rq->avg.load_avg thanks to update_tg_load_avg
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