Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757296AbaJIPag (ORCPT ); Thu, 9 Oct 2014 11:30:36 -0400 Received: from casper.infradead.org ([85.118.1.10]:40805 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751385AbaJIPa2 (ORCPT ); Thu, 9 Oct 2014 11:30:28 -0400 Date: Thu, 9 Oct 2014 17:30:25 +0200 From: Peter Zijlstra To: Vincent Guittot Cc: Ingo Molnar , linux-kernel , Preeti U Murthy , Morten Rasmussen , Kamalesh Babulal , Russell King - ARM Linux , LAK , Rik van Riel , Mike Galbraith , Nicolas Pitre , "linaro-kernel@lists.linaro.org" , Daniel Lezcano , Dietmar Eggemann , Paul Turner , Benjamin Segall Subject: Re: [PATCH v7 2/7] sched: move cfs task on a CPU with higher capacity Message-ID: <20141009153025.GY10832@worktop.programming.kicks-ass.net> References: <1412684017-16595-1-git-send-email-vincent.guittot@linaro.org> <1412684017-16595-3-git-send-email-vincent.guittot@linaro.org> <20141009112352.GO4750@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 09, 2014 at 04:59:36PM +0200, Vincent Guittot wrote: > On 9 October 2014 13:23, Peter Zijlstra wrote: > > On Tue, Oct 07, 2014 at 02:13:32PM +0200, Vincent Guittot wrote: > >> +++ b/kernel/sched/fair.c > >> @@ -5896,6 +5896,18 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group) > >> } > >> > >> /* > >> + * Check whether the capacity of the rq has been noticeably reduced by side > >> + * activity. The imbalance_pct is used for the threshold. > >> + * Return true is the capacity is reduced > >> + */ > >> +static inline int > >> +check_cpu_capacity(struct rq *rq, struct sched_domain *sd) > >> +{ > >> + return ((rq->cpu_capacity * sd->imbalance_pct) < > >> + (rq->cpu_capacity_orig * 100)); > >> +} > >> + > >> +/* > >> * Group imbalance indicates (and tries to solve) the problem where balancing > >> * groups is inadequate due to tsk_cpus_allowed() constraints. > >> * > >> @@ -6567,6 +6579,14 @@ static int need_active_balance(struct lb_env *env) > >> */ > >> if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu) > >> return 1; > >> + > >> + /* > >> + * The src_cpu's capacity is reduced because of other > >> + * sched_class or IRQs, we trig an active balance to move the > >> + * task > >> + */ > >> + if (check_cpu_capacity(env->src_rq, sd)) > >> + return 1; > >> } > > > > So does it make sense to first check if there's a better candidate at > > all? By this time we've already iterated the current SD while trying > > regular load balancing, so we could know this. > > i'm not sure to completely catch your point. > Normally, f_b_g and f_b_q have already looked at the best candidate > when we call need_active_balance. And src_cpu has been elected. > Or i have missed your point ? Yep you did indeed miss my point. So I've always disliked this patch for its arbitrary nature, why unconditionally try and active balance every time there is 'some' RT/IRQ usage, it could be all CPUs are over that arbitrary threshold and we'll end up active balancing for no point. So, since we've already iterated all CPUs in our domain back in update_sd_lb_stats() we could have computed the CFS fraction: 1024 * capacity / capacity_orig for every cpu and collected the min/max of this. Then we can compute if src is significantly (and there I suppose we can indeed use imb) affected compared to others. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/