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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 12590C31E5B for ; Mon, 17 Jun 2019 18:21:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E2E512084D for ; Mon, 17 Jun 2019 18:21:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726669AbfFQSVO (ORCPT ); Mon, 17 Jun 2019 14:21:14 -0400 Received: from mga03.intel.com ([134.134.136.65]:18866 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725764AbfFQSVO (ORCPT ); Mon, 17 Jun 2019 14:21:14 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by orsmga103.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 17 Jun 2019 11:21:13 -0700 X-ExtLoop1: 1 Received: from romley-ivt3.sc.intel.com ([172.25.110.60]) by fmsmga004.fm.intel.com with ESMTP; 17 Jun 2019 11:21:13 -0700 Date: Mon, 17 Jun 2019 11:11:42 -0700 From: Fenghua Yu To: Andy Lutomirski Cc: Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Ashok Raj , Tony Luck , Ravi V Shankar , linux-kernel , x86 Subject: Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state Message-ID: <20190617181142.GA217081@romley-ivt3.sc.intel.com> References: <1559944837-149589-1-git-send-email-fenghua.yu@intel.com> <1559944837-149589-4-git-send-email-fenghua.yu@intel.com> <20190611085410.GT3436@hirez.programming.kicks-ass.net> <0D67CEAC-9710-4ECB-9248-75B48542FF82@amacapital.net> <20190611172717.GC3436@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 17, 2019 at 08:14:44AM -0700, Andy Lutomirski wrote: > On Tue, Jun 11, 2019 at 10:27 AM Peter Zijlstra wrote: > > > > > > (can you, perchance, look at a MUA that isn't 'broken' ?) > > > > On Tue, Jun 11, 2019 at 09:04:30AM -0700, Andy Lutomirski wrote: > > > > > > > > > > On Jun 11, 2019, at 1:54 AM, Peter Zijlstra wrote: > > > > > > > >> On Fri, Jun 07, 2019 at 03:00:35PM -0700, Fenghua Yu wrote: > > > >> C0.2 state in umwait and tpause instructions can be enabled or disabled > > > >> on a processor through IA32_UMWAIT_CONTROL MSR register. > > > >> > > > >> By default, C0.2 is enabled and the user wait instructions result in > > > >> lower power consumption with slower wakeup time. > > > >> > > > >> But in real time systems which require faster wakeup time although power > > > >> savings could be smaller, the administrator needs to disable C0.2 and all > > > >> C0.2 requests from user applications revert to C0.1. > > > >> > > > >> A sysfs interface "/sys/devices/system/cpu/umwait_control/enable_c02" is > > > >> created to allow the administrator to control C0.2 state during run time. > > > > > > > > We already have an interface for applications to convey their latency > > > > requirements (pm-qos). We do not need another magic sys variable. > > > > > > I’m not sure I agree. This isn’t an overall latency request, and > > > setting an absurdly low pm_qos will badly hurt idle power and turbo > > > performance. Also, pm_qos isn’t exactly beautiful. > > > > > > (I speak from some experience. I may be literally the only person to > > > write a driver that listens to dev_pm_qos latency requests. And, in my > > > production box, I directly disable c states instead of messing with > > > pm_qos.) > > > > > > I do wonder whether anyone will ever use this particular control, though. > > > > I agree that pm-qos is pretty terrible; but that doesn't mean we should > > just add random control files all over the place. > > I don't think pm-qos is expressive enough. It seems entirely > reasonable to want to do a C0.1 wait for lower latency *while waiting* > but still want full power-saving idle when not waiting. > > Do we even know what the C0.2 and C0.1 latencies are? And why is this > thing an MSR instead of a flag passed to UMWAIT? I will still keep this sysfs interface in the next version of patches, right? Thanks. -Fenghua