Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751962AbdI0SKE (ORCPT ); Wed, 27 Sep 2017 14:10:04 -0400 Received: from merlin.infradead.org ([205.233.59.134]:45010 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751016AbdI0SKD (ORCPT ); Wed, 27 Sep 2017 14:10:03 -0400 Date: Wed, 27 Sep 2017 20:09:50 +0200 From: Peter Zijlstra To: Dou Liyang Cc: Pasha Tatashin , linux@armlinux.org.uk, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com, john.stultz@linaro.org, sboyd@codeaurora.org, x86@kernel.org, linux-kernel@vger.kernel.org, mingo@redhat.com, tglx@linutronix.de, hpa@zytor.com Subject: Re: [PATCH v6 1/4] sched/clock: interface to allow timestamps early in boot Message-ID: <20170927180950.GD439@worktop> References: <1504116205-355281-1-git-send-email-pasha.tatashin@oracle.com> <1504116205-355281-2-git-send-email-pasha.tatashin@oracle.com> <20170927125857.yvwefpejzskiduwu@hirez.programming.kicks-ass.net> <20170927131003.dxbvu7frcqgtiwaz@hirez.programming.kicks-ass.net> <4a62156c-ea0f-749f-e21b-37919dfda1df@cn.fujitsu.com> <20170927180548.GM17526@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170927180548.GM17526@worktop.programming.kicks-ass.net> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1093 Lines: 24 On Wed, Sep 27, 2017 at 08:05:48PM +0200, Peter Zijlstra wrote: > On Wed, Sep 27, 2017 at 09:52:36PM +0800, Dou Liyang wrote: > > We do not want to do that. Because, we use "notsc" to support Dynamic > > Reconfiguration[1]. > > > > AFAIK, this feature enables hot-add system board which contains CPUs > > and memories. But the CPUs in different board may have different TSCs > > which are not consistent with the TSC from the existing CPUs. If we hot-add > > a board directly, the machine may happen the inconsistency of > > TSC. > > > > We make our effort to specify the same TSC value as existing one through > > hardware and firmware, but it is hard. So we recommend to specify > > "notsc" option in command line for users who want to use Dynamic > > Reconfiguration. > > Oh gawd, that's horrific. And in my book a good reason to kill that > option. That is, even with unsynchronized TSC we're better off using RDTSC. The whole mess in kernel/sched/clock.c is all about getting semi sensible results out of unsynchronized TSC. There really is no reason to artificially kill TSC usage.