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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 C7D00C433E1 for ; Tue, 25 Aug 2020 15:06:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B0EB82076C for ; Tue, 25 Aug 2020 15:06:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726863AbgHYPGV (ORCPT ); Tue, 25 Aug 2020 11:06:21 -0400 Received: from mga05.intel.com ([192.55.52.43]:13714 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726831AbgHYPGU (ORCPT ); Tue, 25 Aug 2020 11:06:20 -0400 IronPort-SDR: iITx/wPJ6QZV2Gb4xyNwAZ9Jy0MraUkbBjKX147zCc8Z9D0a1nEkfXKV5mJvFF0MDjBdf45Qv7 Zas1iSl/5qQA== X-IronPort-AV: E=McAfee;i="6000,8403,9723"; a="240945674" X-IronPort-AV: E=Sophos;i="5.76,353,1592895600"; d="scan'208";a="240945674" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga006.jf.intel.com ([10.7.209.51]) by fmsmga105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Aug 2020 08:06:09 -0700 IronPort-SDR: w4csY4NHOUXkNsjcdUPtlwbZVJ3CFpVIfG5/fxJvpmlLFYLZX1/TmW4eiBBf9fHeSM9Qz1nkb4 QvXEkiLrOpUw== X-IronPort-AV: E=Sophos;i="5.76,353,1592895600"; d="scan'208";a="299118305" Received: from spandruv-mobl.amr.corp.intel.com ([10.251.25.243]) by orsmga006-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Aug 2020 08:06:10 -0700 Message-ID: <1341038e00d965e913ee9f0c7a1e739eadef42f7.camel@linux.intel.com> Subject: Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP value from sysfs From: Srinivas Pandruvada To: "Rafael J. Wysocki" , Artem Bityutskiy Cc: "Rafael J. Wysocki" , Linux PM , LKML , Doug Smythies Date: Tue, 25 Aug 2020 08:06:09 -0700 In-Reply-To: References: <4169555.5IIHXK4Dsd@kreacher> <2064342.aRc67yb0pC@kreacher> <61ea43fce7dd8700d94f12236a86ffec6f76a898.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.34.4 (3.34.4-1.fc31) 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 Tue, 2020-08-25 at 16:51 +0200, Rafael J. Wysocki wrote: > On Tue, Aug 25, 2020 at 8:20 AM Artem Bityutskiy > wrote: > > 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. > > If the write is successful (ie. no errors returned and the value > returned is equal to the number of written characters), the kernel > *has* accepted the written value, but it may not have taken effect. > These are two different things. > > The written value may take an effect immediately or it may take an > effect later, depending on the current configuration etc. If you > don't see the effect of it immediately, it doesn't matter that there > was a failure of some sort. > > > 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 do you want it to fail then? > > > Why caching the value and making my script _think_ it succeeded is > > a good idea. > > Because when you change the scaling algorithm or the driver's > operation mode, the value you have written will take effect. > > In this particular case it is explained in the driver documentation > that the performance scaling algorithm in the active mode overrides > the sysfs value and that's the only case when it can be overridden. > So whatever you write to this attribute will not take effect > immediately anyway, but it may take an effect later. In some cases without even changing active/passive this is happening when there was some error previously. For example: #cat energy_performance_preference 127 [root@otcpl-perf-test-skx-i9 cpufreq]# rdmsr -p 1 0x774 8000ff00 I think we should show reality. In mode change can be a special case and use the stored value to restore in new mode. Thanks, Srinivas > > In other words, in my usage scenarios at list, I prefer kernel > > telling > > the true EPP value, not some "cached, but not used" value. > > An alternative is to fail writes to energy_performance_preference if > the driver works in the active mode and the scaling algorithm for the > scaling CPU is performance and *then* to make reads from it return > the > value in the register. > > Accepting a write and returning a different value in a subsequent > read > is confusing. > > Thanks!