From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9E163C4332F for ; Wed, 9 Nov 2022 10:44:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231268AbiKIKoc (ORCPT ); Wed, 9 Nov 2022 05:44:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56836 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231229AbiKIKoR (ORCPT ); Wed, 9 Nov 2022 05:44:17 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 2BC99AE72 for ; Wed, 9 Nov 2022 02:44:16 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0AAE01FB; Wed, 9 Nov 2022 02:44:22 -0800 (PST) Received: from [192.168.178.6] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 297063F534; Wed, 9 Nov 2022 02:44:14 -0800 (PST) Message-ID: <107bf66d-8924-c287-125f-10a648c7b701@arm.com> Date: Wed, 9 Nov 2022 11:43:58 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v2 9/9] sched/fair: Consider capacity inversion in util_fits_cpu() Content-Language: en-US To: Qais Yousef , Valentin Schneider Cc: Ingo Molnar , "Peter Zijlstra (Intel)" , Vincent Guittot , linux-kernel@vger.kernel.org, Xuewen Yan , Lukasz Luba , Wei Wang , Jonathan JMChen , Hank References: <20220804143609.515789-1-qais.yousef@arm.com> <20220804143609.515789-10-qais.yousef@arm.com> <20221105204141.3tno6fzuh536ye4e@airbuntu> From: Dietmar Eggemann In-Reply-To: <20221105204141.3tno6fzuh536ye4e@airbuntu> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org - Qais Yousef On 05/11/2022 21:41, Qais Yousef wrote: > On 11/04/22 17:35, Valentin Schneider wrote: >> On 04/08/22 15:36, Qais Yousef wrote: [...] >> IIUC the rq->cpu_capacity_inverted computation in update_cpu_capacity() can be >> summarised as: >> >> - If there is a PD with equal cap_orig, but higher effective (orig - thermal) >> capacity >> OR >> there is a PD with pd_cap_orig > cpu_effective_cap: >> rq->cpu_capacity_inverted = capacity_orig - thermal_load_avg(rq) >> >> - Else: >> rq->cpu_capacity_inverted = 0 >> >> Then, the code above uses either rq->cpu_capacity_inverted if it is >> non-zero, otherwise: >> >> capacity_orig - arch_scale_thermal_pressure(cpu); >> >> Why use average thermal pressure in one case, and use instantaneous >> thermal pressure in the other? > > There was a big debate on [1] about using avg vs instantaneous. > > I used avg for detecting inversion to be consistent with using average in in > scale_rt_capacity(). I didn't want the inversion state to be flipping too > quickly too. > > I used the instantaneous in the other check based on that discussion. It seemed > using the average is hurtful when for example the medium drops an OPP and by > not reacting quickly at wake up we lose the chance to place it on a big; which > if my memory didn't fail me is what Xuewen was seeing. > > [1] https://lore.kernel.org/lkml/24631a27-42d9-229f-d9b0-040ac993b749@arm.com/ > >> >> Can't we get rid of rq->cpu_capacity_inverted and replace this whole thing >> with an unconditional >> >> capacity_orig_thermal = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu)); >> >> ? > > I can't see how we end up with equivalent behavior then. Or address the > concerns raised by Xuewen and Lukasz on the RT thread in regards to avg vs > instantaneous. > > Specifically, if we don't use the new rq->cpu_capacity_inverted we can't handle > the case where the task is requesting to run at maximum performance but a small > drop in thermal pressure means it won't fit anywhere. That PD is the best fit > until it hits an inversion. > > Originally I wanted to defer handling thermal pressure into a different series. > But Vincent thought it's better to handle it now. We want more data points from > more systems tbh. But I think what we have now is still a good improvement over > what we had before. I can't see the rationale in using: !inversion: `cap_orig - instantaneous thermal pressure` inversion: `cap_orig - PELT thermal pressure` I can see that there was a lot of discussion on this topic but hardly any agreement IMHO. AFAICS, the 2 capacity inversion patches just appeared in v2 and haven't seen any review yet I'm afraid. [...]