From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752913AbbIJLCt (ORCPT ); Thu, 10 Sep 2015 07:02:49 -0400 Received: from foss.arm.com ([217.140.101.70]:33481 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750781AbbIJLCq (ORCPT ); Thu, 10 Sep 2015 07:02:46 -0400 Date: Thu, 10 Sep 2015 12:06:39 +0100 From: Morten Rasmussen To: bsegall@google.com Cc: Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Steve Muckle , "mingo@redhat.com" , "daniel.lezcano@linaro.org" , "yuyang.du@intel.com" , "mturquette@baylibre.com" , "rjw@rjwysocki.net" , Juri Lelli , "sgurrappadi@nvidia.com" , "pang.xunlei@zte.com.cn" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig Message-ID: <20150910110638.GE27098@e105550-lin.cambridge.arm.com> References: <55E8DD00.2030706@linaro.org> <55EDAF43.30500@arm.com> <55EDDD5A.70904@arm.com> <20150908122606.GH3644@twins.programming.kicks-ass.net> <20150908125205.GW18673@twins.programming.kicks-ass.net> <20150908143157.GA27098@e105550-lin.cambridge.arm.com> <20150908153300.GK3644@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 09, 2015 at 03:23:43PM -0700, bsegall@google.com wrote: > Peter Zijlstra writes: > > > On Tue, Sep 08, 2015 at 03:31:58PM +0100, Morten Rasmussen wrote: > >> On Tue, Sep 08, 2015 at 02:52:05PM +0200, Peter Zijlstra wrote: > >> > > Tricky that, LOAD_AVG_MAX very much relies on the unit being 1<<10. > >> > >> I don't get why LOAD_AVG_MAX relies on the util_avg shifting being > >> 1<<10, it is just the sum of the geometric series and the upper bound of > >> util_sum? > > > > It needs a 1024, it might just have been the 1024 ns we use a period > > instead of the scale unit though. > > > > The LOAD_AVG_MAX is the number where adding a next element to the series > > doesn't change the result anymore, so scaling it up will allow more > > significant elements to the series before we bottom out, which is the _N > > thing. > > > > Yes, as the comments say, the 1024ns unit is arbitrary (and is an > average of not-quite-microseconds instead of just nanoseconds to allow > more bits to load.weight when we multiply load.weight by this number). > In fact there are two arbitrary 1024 units here, which are technically > unrelated and are both unrelated to SCHED_LOAD_RESOLUTION/etc - we > operate on units of almost-microseconds and we also do decays every > almost-millisecond. > > There appears to be a bunch of confusion in the current code around > util_sum/util_avg which appears to using SCHED_LOAD_SCALE > for a fixed-point percentage or something, which is at least reasonable, > but is initializing it as scale_load_down(SCHED_LOAD_SCALE), which > results in either initializing as 100% or .1% depending on RESOLUTION. > This'll get clobbered on first update, but if it needs to be > initialized, it should either get initialized to something sane or at > least consistent. This is what I thought too. The whole geometric series math is completely independent of the scale used for priority in load_avg and the fixed point shifting used for util_avg. > load_sum/load_avg appear to be scale_load_down()ed properly, and appear > to be used as such at a quick glance. I don't think shifting by SCHED_LOAD_SHIFT in __update_load_avg() is right: sa->util_avg = (sa->util_sum << SCHED_LOAD_SHIFT) / LOAD_AVG_MAX; util_avg is initialized to low resolution (>> SCHED_LOAD_RESOLUTION): sa->util_avg = scale_load_down(SCHED_LOAD_SCALE); so it appear to be intended to be using low resolution like load_avg (weight is scaled down before it is passed into __update_load_avg()), but util_avg is shifted up to high resolution. It should be: sa->util_avg = (sa->util_sum << (SCHED_LOAD_SHIFT - SCHED_LOAD_SHIFT)) / LOAD_AVG_MAX; to be consistent.