Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752588AbdGMPNe (ORCPT ); Thu, 13 Jul 2017 11:13:34 -0400 Received: from mga04.intel.com ([192.55.52.120]:13863 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752543AbdGMPNc (ORCPT ); Thu, 13 Jul 2017 11:13:32 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.40,354,1496127600"; d="scan'208";a="1195005884" Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods To: Peter Zijlstra 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 References: <20170710172705.GA3441@tassilo.jf.intel.com> <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> From: "Li, Aubrey" Message-ID: <4a577bd6-20b1-abb6-2153-f9870f0a721e@linux.intel.com> Date: Thu, 13 Jul 2017 23:13:28 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170713145311.z4zxlyd2dospeoqg@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1565 Lines: 36 On 2017/7/13 22:53, Peter Zijlstra wrote: > On Thu, Jul 13, 2017 at 10:48:55PM +0800, Li, Aubrey wrote: > >> - totally from arch_cpu_idle_enter entry to arch_cpu_idle_exit return costs >> 9122ns - 15318ns. >> ---- In this period(arch idle), rcu_idle_enter costs 1985ns - 2262ns, rcu_idle_exit >> costs 1813ns - 3507ns >> >> Besides RCU, > > So Paul wants more details on where RCU hurts so we can try to fix. > If we can call RCU idle enter/exit after tick is really stopped, instead of call it every idle, I think it's fine. Then we can skip stopping tick if we need fast idle. >> the period includes c-state selection on X86, a few timestamp updates >> and a few computations in menu governor. Also, deep HW-cstate latency can be up >> to 100+ microseconds, even if the system is very busy, CPU still has chance to enter >> deep cstate, which I guess some outburst workloads are not happy with it. >> >> That's my major concern without a fast idle path. > > 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. Thanks, -Aubrey