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[23.128.96.18]) by mx.google.com with ESMTP id z11si1341377ejw.610.2020.08.25.08.15.45; Tue, 25 Aug 2020 08:16:08 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726432AbgHYPPL (ORCPT + 99 others); Tue, 25 Aug 2020 11:15:11 -0400 Received: from mail-ot1-f68.google.com ([209.85.210.68]:36044 "EHLO mail-ot1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726113AbgHYPOv (ORCPT ); Tue, 25 Aug 2020 11:14:51 -0400 Received: by mail-ot1-f68.google.com with SMTP id x24so10655482otp.3; Tue, 25 Aug 2020 08:14:51 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=4SuxOmAQqZZFaDvKNEg/FnVPjE4bxt3cvqZyJwNFitw=; b=U5QfGkHm4UygzPoNmhFGBlXRf9C24r7otGtxmHg/Y3aoPqtdGS6qpdWaDVz2heWDpz 8MjjKvIj3ODMrU6JCk1lD6QsXowDu6d1rlsQsxytGXebH0cZNbgq0YQs3XOx7l2PSmSs 1Yd6UT8DqLvHpkEpGtn5IpBmw/l2jcw3nTzFslBdPvvaMYFkzsnF2GuoPARkFCqQI2PE EjUevCC3cCmxyOvfRGcZ9vmvSn4R+mPFl/c9NeXqN7YSnR4B1bBLITQZaLe3zuqqoc10 D2S+Efzy1seljYwruwTLfuvsqf9XPHlRYaG0XMVjgJaL0EOPDaOsX77TLWmbA8+AcDuh 0zyg== X-Gm-Message-State: AOAM5319q6ZYL7buiUmcVvZ5+EbPIrj78yMdmZ6O3A1hI3Q4HN8Cldy2 zMh8kLdXhdpdwzwwfDzmQPC239h7kuEuufqaef3krSES X-Received: by 2002:a9d:5c06:: with SMTP id o6mr7145886otk.262.1598368490533; Tue, 25 Aug 2020 08:14:50 -0700 (PDT) MIME-Version: 1.0 References: <4169555.5IIHXK4Dsd@kreacher> <2064342.aRc67yb0pC@kreacher> <61ea43fce7dd8700d94f12236a86ffec6f76a898.camel@gmail.com> <1341038e00d965e913ee9f0c7a1e739eadef42f7.camel@linux.intel.com> In-Reply-To: <1341038e00d965e913ee9f0c7a1e739eadef42f7.camel@linux.intel.com> From: "Rafael J. Wysocki" Date: Tue, 25 Aug 2020 17:14:39 +0200 Message-ID: Subject: Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP value from sysfs To: Srinivas Pandruvada Cc: "Rafael J. Wysocki" , Artem Bityutskiy , "Rafael J. Wysocki" , Linux PM , LKML , Doug Smythies Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > 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". Cheers!