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[2620:137:e000::1:20]) by mx.google.com with ESMTP id ls11-20020a17090b350b00b002502a6bb94dsi2043133pjb.179.2023.05.05.08.49.25; Fri, 05 May 2023 08:49:38 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=arm.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232747AbjEEP0l (ORCPT + 99 others); Fri, 5 May 2023 11:26:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56484 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232955AbjEEP0Y (ORCPT ); Fri, 5 May 2023 11:26:24 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 14C9E1A107; Fri, 5 May 2023 08:25:18 -0700 (PDT) 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 292E51FB; Fri, 5 May 2023 08:26:02 -0700 (PDT) Received: from e130256.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 E80D13F5A1; Fri, 5 May 2023 08:25:16 -0700 (PDT) From: Hongyan Xia To: Jonathan Corbet Cc: Hongyan Xia , Qais Yousef , Vincent Guittot , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] sched/documentation: elaborate on uclamp limitations Date: Fri, 5 May 2023 16:24:39 +0100 Message-Id: <20230505152440.142265-1-hongyan.xia2@arm.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 X-Spam-Status: No, score=-4.2 required=5.0 tests=BAYES_00,RCVD_IN_DNSWL_MED, SPF_HELO_NONE,SPF_NONE,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1765068763502985405?= X-GMAIL-MSGID: =?utf-8?q?1765069845547572065?= The story in 5.2 about util_avg abruptly jumping from 300 when Fmax/Fmin == 3 to 1024 when Fmax/Fmin == 4 hides some details about how clock_pelt works behind the scenes. Explicitly mention it to make it easier for readers to follow. Signed-off-by: Hongyan Xia Cc: Qais Yousef Cc: Vincent Guittot --- Documentation/scheduler/sched-util-clamp.rst | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/Documentation/scheduler/sched-util-clamp.rst b/Documentation/scheduler/sched-util-clamp.rst index 74d5b7c6431d..524df07bceba 100644 --- a/Documentation/scheduler/sched-util-clamp.rst +++ b/Documentation/scheduler/sched-util-clamp.rst @@ -669,6 +669,19 @@ but not proportional to Fmax/Fmin. p0->util_avg = 300 + small_error +The reason why util_avg is around 300 even though it runs for 900 at Fmin is: +Although running at Fmin reduces the rate of rq_clock_pelt() to 1/3 thus +accumulates util_sum at 1/3 of the rate at Fmax, the clock period +(rq_clock_pelt() now minus previous rq_clock_pelt()) in: + +:: + + util_sum / clock period = util_avg + +does not shrink to 1/3, since rq->clock_pelt is periodically synchronized with +rq->clock_task as long as there's idle time. As a result, we get util_avg of +about 300, not 900. + Now if the ratio of Fmax/Fmin is 4, the maximum value becomes: :: @@ -682,6 +695,10 @@ this happens, then the _actual_ util_avg will become: p0->util_avg = 1024 +This is because rq->clock_pelt is no longer synchronized with the task clock. +The clock period therefore is proportionally shrunk by the same ratio of +(Fmax/Fmin), giving us a maximal util_avg of 1024. + If task p1 wakes up on this CPU, which have: ::