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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 E69AEECE565 for ; Tue, 18 Sep 2018 07:52:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7B6D42146D for ; Tue, 18 Sep 2018 07:52:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7B6D42146D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linutronix.de Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729134AbeIRNYB (ORCPT ); Tue, 18 Sep 2018 09:24:01 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:57014 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728605AbeIRNYB (ORCPT ); Tue, 18 Sep 2018 09:24:01 -0400 Received: from p5492e4c1.dip0.t-ipconnect.de ([84.146.228.193] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1g2AoB-0004GC-Gg; Tue, 18 Sep 2018 09:52:31 +0200 Date: Tue, 18 Sep 2018 09:52:26 +0200 (CEST) From: Thomas Gleixner To: John Stultz cc: Andy Lutomirski , LKML , X86 ML , Peter Zijlstra , Matt Rickard , Stephen Boyd , Florian Weimer , "K. Y. Srinivasan" , Vitaly Kuznetsov , devel@linuxdriverproject.org, Linux Virtualization , Paolo Bonzini , Arnd Bergmann , Juergen Gross Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case In-Reply-To: Message-ID: References: <20180914125006.349747096@linutronix.de> <20180914125118.909646643@linutronix.de> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 17 Sep 2018, John Stultz wrote: > On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski wrote: > > Also, I'm not entirely convinced that this "last" thing is needed at > > all. John, what's the scenario under which we need it? > > So my memory is probably a bit foggy, but I recall that as we > accelerated gettimeofday, we found that even on systems that claimed > to have synced TSCs, they were actually just slightly out of sync. > Enough that right after cycles_last had been updated, a read on > another cpu could come in just behind cycles_last, resulting in a > negative interval causing lots of havoc. > > So the sanity check is needed to avoid that case. Your memory serves you right. That's indeed observable on CPUs which lack TSC_ADJUST. @Andy: Welcome to the wonderful world of TSC. Thanks, tglx