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 85A8CC433DF for ; Tue, 25 Aug 2020 15:27:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6E1EB207BC for ; Tue, 25 Aug 2020 15:27:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726351AbgHYP1D (ORCPT ); Tue, 25 Aug 2020 11:27:03 -0400 Received: from mga12.intel.com ([192.55.52.136]:17413 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726294AbgHYP1B (ORCPT ); Tue, 25 Aug 2020 11:27:01 -0400 IronPort-SDR: QuX9GVzsDL7x1TT5jFypR2VNpP5B2g9hRJ4pcl8BNOwE1ChfbgR/UGZhVo36+VT1WjjcCt9dqv OYtfjBSqqMWw== X-IronPort-AV: E=McAfee;i="6000,8403,9723"; a="135682155" X-IronPort-AV: E=Sophos;i="5.76,353,1592895600"; d="scan'208";a="135682155" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Aug 2020 08:27:00 -0700 IronPort-SDR: lYqApXSFkS1xEINspjgKHsy0I17SaykBZsH8rFck1MqANvUym6lCEQMnTH6I0tJ24bmKvz+wvZ ZnIuVhbH14Zg== X-IronPort-AV: E=Sophos;i="5.76,353,1592895600"; d="scan'208";a="474390922" Received: from spandruv-mobl.amr.corp.intel.com ([10.251.25.243]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Aug 2020 08:27:00 -0700 Message-ID: Subject: Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP value from sysfs From: Srinivas Pandruvada To: "Rafael J. Wysocki" Cc: Artem Bityutskiy , "Rafael J. Wysocki" , Linux PM , LKML , Doug Smythies Date: Tue, 25 Aug 2020 08:26:59 -0700 In-Reply-To: References: <4169555.5IIHXK4Dsd@kreacher> <2064342.aRc67yb0pC@kreacher> <61ea43fce7dd8700d94f12236a86ffec6f76a898.camel@gmail.com> <1341038e00d965e913ee9f0c7a1e739eadef42f7.camel@linux.intel.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 17:14 +0200, Rafael J. Wysocki wrote: > On Tue, Aug 25, 2020 at 5:06 PM Srinivas Pandruvada > wrote: > > On Tue, 2020-08-25 at 16:51 +0200, Rafael J. Wysocki wrote: > > > On Tue, Aug 25, 2020 at 8:20 AM Artem Bityutskiy < > > > dedekind1@gmail.com > > > > 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. > > OK, so I'll make it fail on attempts to change the EPP from 0 > (performance) in the active mode with the performance "governor". > Here the scaling governor is powersave. # cat scaling_governor powersave Thanks, Srinivas > Cheers!