From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752516AbdJFNDJ (ORCPT ); Fri, 6 Oct 2017 09:03:09 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:41639 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752035AbdJFNDH (ORCPT ); Fri, 6 Oct 2017 09:03:07 -0400 Date: Fri, 6 Oct 2017 15:02:57 +0200 From: Peter Zijlstra To: Dietmar Eggemann Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, tj@kernel.org, josef@toxicpanda.com, torvalds@linux-foundation.org, vincent.guittot@linaro.org, efault@gmx.de, pjt@google.com, clm@fb.com, morten.rasmussen@arm.com, bsegall@google.com, yuyang.du@intel.com Subject: Re: [PATCH -v2 15/18] sched/fair: Align PELT windows between cfs_rq and its se Message-ID: <20171006130257.l4jekk5bvme3pcma@hirez.programming.kicks-ass.net> References: <20170901132059.342024223@infradead.org> <20170901132748.738108335@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 04, 2017 at 08:27:01PM +0100, Dietmar Eggemann wrote: > On 01/09/17 14:21, Peter Zijlstra wrote: > > The PELT _sum values are a saw-tooth function, dropping on the decay > > edge and then growing back up again during the window. > > > > When these window-edges are not aligned between cfs_rq and se, we can > > have the situation where, for example, on dequeue, the se decays > > first. > > > > Its _sum values will be small(er), while the cfs_rq _sum values will > > still be on their way up. Because of this, the subtraction: > > cfs_rq->avg._sum -= se->avg._sum will result in a positive value. This > > will then, once the cfs_rq reaches an edge, translate into its _avg > > value jumping up. > > > > This is especially visible with the runnable_load bits, since they get > > added/subtracted a lot. > > > > Signed-off-by: Peter Zijlstra (Intel) > > --- > > kernel/sched/fair.c | 45 +++++++++++++++++++++++++++++++-------------- > > 1 file changed, 31 insertions(+), 14 deletions(-) > > [...] > > > @@ -3644,7 +3634,34 @@ update_cfs_rq_load_avg(u64 now, struct c > > */ > > static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) > > { > > + u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib; > > + > > + /* > > + * When we attach the @se to the @cfs_rq, we must align the decay > > + * window because without that, really weird and wonderful things can > > + * happen. > > + * > > + * XXX illustrate > > + */ > > se->avg.last_update_time = cfs_rq->avg.last_update_time; > > + se->avg.period_contrib = cfs_rq->avg.period_contrib; > > + > > + /* > > + * Hell(o) Nasty stuff.. we need to recompute _sum based on the new > > + * period_contrib. This isn't strictly correct, but since we're > > + * entirely outside of the PELT hierarchy, nobody cares if we truncate > > + * _sum a little. > > + */ > > + se->avg.util_sum = se->avg.util_avg * divider; > > + > > + se->avg.load_sum = divider; > > + if (se_weight(se)) { > > + se->avg.load_sum = > > + div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se)); > > + } > > Can scale_load_down(se->load.weight) ever become 0 here? Yeah, don't see why not.