Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752614AbdGMS2o (ORCPT ); Thu, 13 Jul 2017 14:28:44 -0400 Received: from merlin.infradead.org ([205.233.59.134]:50700 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751337AbdGMS2n (ORCPT ); Thu, 13 Jul 2017 14:28:43 -0400 Date: Thu, 13 Jul 2017 20:28:20 +0200 From: Peter Zijlstra To: "Li, Aubrey" Cc: Andi Kleen , Frederic Weisbecker , Christoph Lameter , Aubrey Li , tglx@linutronix.de, len.brown@intel.com, rjw@rjwysocki.net, tim.c.chen@linux.intel.com, arjan@linux.intel.com, paulmck@linux.vnet.ibm.com, yang.zhang.wz@gmail.com, x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods Message-ID: <20170713182820.sn3fjitnd3mca27p@hirez.programming.kicks-ass.net> References: <20170711094157.5xcwkloxnjehieqv@hirez.programming.kicks-ass.net> <20170711160926.GA18805@lerouge> <20170711163422.etydkhhtgfthpfi5@hirez.programming.kicks-ass.net> <496d4921-5768-cd1e-654b-38630b7d2e13@linux.intel.com> <20170712083410.ualmvnvzoohyami5@hirez.programming.kicks-ass.net> <20170712213240.GE3441@tassilo.jf.intel.com> <20170713083649.febfflfl5hafkko5@hirez.programming.kicks-ass.net> <16e12e23-6b28-f174-7c4b-4d719225cd3b@linux.intel.com> <20170713145311.z4zxlyd2dospeoqg@hirez.programming.kicks-ass.net> <4a577bd6-20b1-abb6-2153-f9870f0a721e@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4a577bd6-20b1-abb6-2153-f9870f0a721e@linux.intel.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 824 Lines: 16 On Thu, Jul 13, 2017 at 11:13:28PM +0800, Li, Aubrey wrote: > On 2017/7/13 22:53, Peter Zijlstra wrote: > > Fixing C-state selection by creating an alternative idle path sounds so > > very wrong. > > This only happens on the arch which has multiple hardware idle cstates, like > Intel's processor. As long as we want to support multiple cstates, we have to > make a selection(with cost of timestamp update and computation). That's fine > in the normal idle path, but if we want a fast idle switch, we can make a > tradeoff to use a low-latency one directly, that's why I proposed a fast idle > path, so that we don't need to mix fast idle condition judgement in both idle > entry and idle exit path. That doesn't make sense. If you can decide to pick a shallow C state in any way, you can fix the general selection too.