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=-3.3 required=3.0 tests=BAYES_00,DKIM_ADSP_CUSTOM_MED, FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM,FREEMAIL_REPLYTO_END_DIGIT, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 96D56C433E1 for ; Tue, 25 Aug 2020 06:20:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6D43E2083B for ; Tue, 25 Aug 2020 06:20:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726015AbgHYGUi (ORCPT ); Tue, 25 Aug 2020 02:20:38 -0400 Received: from mga14.intel.com ([192.55.52.115]:16543 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725946AbgHYGUg (ORCPT ); Tue, 25 Aug 2020 02:20:36 -0400 IronPort-SDR: 2SY5D2WdYCbC8VUEUoViwPQrr8PhXFqechQzgJQXYvqMO6fXQvXly0esxqBfVpjHo8pFwLVtzu qG6OlvhZ+c/Q== X-IronPort-AV: E=McAfee;i="6000,8403,9723"; a="155310052" X-IronPort-AV: E=Sophos;i="5.76,351,1592895600"; d="scan'208";a="155310052" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Aug 2020 23:20:36 -0700 IronPort-SDR: euuB1noOm8pHoIwZaNBXb6cEr0hO6JaO7Xtvab0eeJ4IVKHJdNcFVXEUbTQCsT4/yDl6vz7790 cs3BCf/K/O5A== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.76,351,1592895600"; d="scan'208";a="336405671" Received: from linux.intel.com ([10.54.29.200]) by FMSMGA003.fm.intel.com with ESMTP; 24 Aug 2020 23:20:35 -0700 Received: from abityuts-desk1.fi.intel.com (abityuts-desk1.fi.intel.com [10.237.72.186]) by linux.intel.com (Postfix) with ESMTP id 656285805EB; Mon, 24 Aug 2020 23:20:34 -0700 (PDT) Message-ID: <61ea43fce7dd8700d94f12236a86ffec6f76a898.camel@gmail.com> Subject: Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP value from sysfs From: Artem Bityutskiy Reply-To: dedekind1@gmail.com To: "Rafael J. Wysocki" , Linux PM Cc: Srinivas Pandruvada , LKML , Doug Smythies Date: Tue, 25 Aug 2020 09:20:33 +0300 In-Reply-To: <2064342.aRc67yb0pC@kreacher> References: <4169555.5IIHXK4Dsd@kreacher> <2064342.aRc67yb0pC@kreacher> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.32.5 (3.32.5-1.fc30) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-pm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org On Mon, 2020-08-24 at 19:42 +0200, Rafael J. Wysocki wrote: > From: "Rafael J. Wysocki" > > Make the energy_performance_preference policy attribute in sysfs > always return the last EPP value written to it instead of the one > currently in the HWP Request MSR to avoid possible confusion when > the performance scaling algorithm is used in the active mode with > HWP enabled (in which case the EPP is forced to 0 regardless of > what value it has been set to via sysfs). Why is this a good idea, I wonder. If there was a prior discussion, please, point to it. The general approach to changing settings via sysfs is often like this: 1. Write new value. 2. Read it back and verify that it is the same. Because there is no better way to verify that the kernel "accepted" the value. Let's say I write 'balanced' to energy_performance_preference. I read it back, and it contains 'balanced', so I am happy, I trust the kernel changed EPP to "balanced". If the kernel, in fact, uses something else, I want to know about it and have my script fail. Why caching the value and making my script _think_ it succeeded is a good idea. In other words, in my usage scenarios at list, I prefer kernel telling the true EPP value, not some "cached, but not used" value. Artem.