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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4377FC433ED for ; Fri, 7 May 2021 15:38:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 05409610A0 for ; Fri, 7 May 2021 15:38:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237984AbhEGPjg (ORCPT ); Fri, 7 May 2021 11:39:36 -0400 Received: from foss.arm.com ([217.140.110.172]:34370 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231297AbhEGPje (ORCPT ); Fri, 7 May 2021 11:39:34 -0400 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 D464811B3; Fri, 7 May 2021 08:38:34 -0700 (PDT) Received: from e120877-lin.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 329933F73B; Fri, 7 May 2021 08:38:33 -0700 (PDT) Date: Fri, 7 May 2021 16:38:28 +0100 From: Vincent Donnefort To: Dietmar Eggemann Cc: Xuewen Yan , mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, linux-kernel@vger.kernel.org, zhang.lyra@gmail.com, xuewyan@foxmail.com Subject: Re: [PATCH v2] sched/pelt: Keep UTIL_AVG_UNCHANGED flag in sync when calculating last_enqueued_diff Message-ID: <20210507153827.GA176408@e120877-lin.cambridge.arm.com> References: <20210507112031.23073-1-xuewen.yan94@gmail.com> <20210507123528.GA106818@e120877-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 07, 2021 at 03:36:47PM +0200, Dietmar Eggemann wrote: > On 07/05/2021 14:35, Vincent Donnefort wrote: > > On Fri, May 07, 2021 at 07:20:31PM +0800, Xuewen Yan wrote: > >> From: Xuewen Yan > >> > >> Last_enqueued_diff's meaning: "diff = util_est.enqueued(p) - task_util(p)". > >> When calculating last_enqueued_diff, we add UTIL_AVG_UNCHANGED flag, which > >> is the LSB, for task_util, but don't add the flag for util_est.enqueued. > >> However the enqueued's flag had been cleared when the task util changed. > >> As a result, we add +1 to the diff, this is therefore reducing slightly > >> UTIL_EST_MARGIN. > > > > Unless I miss something it actually depends on the situation, doesn't it? > > > > if ue.enqueued > task_util(), we remove 1 from the diff => UTIL_EST_MARGIN + 1 > > > > if ue.enqueued < task_util(), we add 1 to the diff => UTIL_EST_MARGIN -1 > > I get: > > ue.enqueued & UTIL_AVG_UNCHANGED = 0 > > last_enqueued_diff = ue.enqueued_old - ue.enqueued_new > > last_enqueued_diff = (ue.enqueued | UTIL_AVG_UNCHANGED) - (task_util(p) | UTIL_AVG_UNCHANGED) > > ^^^^^^^^^^^^^^^^^^^^ > added by patch > > > ue.enqueued_old didn't have the flag, otherwise would return earlier > > task_util(p) could have the LSB set but we just set it to make sure it's set > > So last_enqueued_diff is 1 larger. But we take the abs() of last_enqueued_diff. If we consider the following examples: enqueued_old = 5, enqueued_new = 9 diff = 5 - (9 + 1) => 5 - 10 => -5 enqueued_old = 9, enqueued_new = 5 diff = 9 - (5 + 1) => 9 - 6 => 3 In both cases the delta is supposed to be 4. But in the first case we end-up with 5. In the second, we end-up with 3. That's why I meant the effect on the diff depends on who's greater, ue.enqueued or task_util(). > > But UTIL_EST_MARGIN stays `SCHED_CAPACITY_SCALE / 100` ? > > Did I miss something here? But the only purpose of the diff is the comparison against the margin. So +/-1 in the diff ends-up being the same as doing the opposite for the margin. All in all, the missing flag ends-up by modifying UTIL_EST_MARGIN by -1 or +1. -- Vincent