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 A90E4C43142 for ; Sat, 23 Jun 2018 16:51:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5B34224C8C for ; Sat, 23 Jun 2018 16:51:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5B34224C8C 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 S1751824AbeFWQvE (ORCPT ); Sat, 23 Jun 2018 12:51:04 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:43821 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751598AbeFWQvD (ORCPT ); Sat, 23 Jun 2018 12:51:03 -0400 Received: from p4fea482e.dip0.t-ipconnect.de ([79.234.72.46] helo=nanos.glx-home) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1fWljo-0007A8-68; Sat, 23 Jun 2018 18:50:12 +0200 Date: Sat, 23 Jun 2018 18:50:10 +0200 (CEST) From: Thomas Gleixner To: Pavel Tatashin cc: steven.sistare@oracle.com, daniel.m.jordan@oracle.com, 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, hpa@zytor.com, douly.fnst@cn.fujitsu.com, peterz@infradead.org, prarit@redhat.com, feng.tang@intel.com, pmladek@suse.com, gnomes@lxorguk.ukuu.org.uk, linux-s390@vger.kernel.org Subject: Re: [PATCH v12 09/11] x86/tsc: prepare for early sched_clock In-Reply-To: <20180621212518.19914-10-pasha.tatashin@oracle.com> Message-ID: References: <20180621212518.19914-1-pasha.tatashin@oracle.com> <20180621212518.19914-10-pasha.tatashin@oracle.com> 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 Thu, 21 Jun 2018, Pavel Tatashin wrote: > We will change sched_clock() to be called early. Why is this relevant? Does the issue only appear with that change? > But, during boot sched_clock() changes its output without notifying us > about the change of clock source. Why would the sched clock change notify _US_? Can you please try to write your changelog in factual technical terms without impersonating the system/code? > This happens in tsc_init(), when static_branch_enable(&__use_tsc) is > called. > > native_sched_clock() changes from outputing jiffies to reading tsc, but > sched is not notified in anyway. So, to preserve the continoutity in this sched? -EMAKESNOSENSE There is no notification mechanism and it's not required. > place we add the offset of sched_clock() to the calculation of cyc2ns. s/we// See Documentation/process/submitting-patches.rst Describe your changes in imperative mood, e.g. "make xyzzy do frotz" instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy to do frotz", as if you are giving orders to the codebase to change its behaviour. > Without this change, the output would look like this: Would? It looks exactly like this or why would you try to fix it? > > [ 0.004000] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1 > [ 0.009000] tsc: Fast TSC calibration using PIT > [ 0.010000] tsc: Detected 3192.137 MHz processor > [ 0.011000] clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x2e03465ceb2, max_idle_ns: 440795259855 ns > > static_branch_enable(__use_tsc) is called, and timestamps became precise > but reduced: > > [ 0.002233] Calibrating delay loop (skipped), value calculated using timer frequency.. 6384.27 BogoMIPS (lpj=3192137) > [ 0.002516] pid_max: default: 32768 minimum: 301 Let me give you an example: When tsc_init() enables the usage of TSC for sched_clock() the initialization of the tsc sched clock conversion data starts from zero and not from the current jiffies based sched_clock() value. This makes the timestamps jump backwards: [ 0.010000] tsc: Detected 3192.137 MHz processor [ 0.011000] clocksource: tsc-early: mask: ... [ 0.002233] Calibrating delay loop (skipped), .... To address this, extend set_cyc2ns_scale() with an argument to base the cyc2ns offset on the current sched_clock() value. During run time this offset is 0 as the cyc2ns offset is based on the TSC sched_clock() itself. See? Precise and pure technical. No we/us/would/ and no irrelevant information. > Signed-off-by: Pavel Tatashin > --- > arch/x86/kernel/tsc.c | 15 +++++++++------ > 1 file changed, 9 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c > index 186395041725..654a01cc0358 100644 > --- a/arch/x86/kernel/tsc.c > +++ b/arch/x86/kernel/tsc.c > @@ -133,7 +133,9 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc) > return ns; > } > > -static void set_cyc2ns_scale(unsigned long khz, int cpu, unsigned long long tsc_now) > +static void set_cyc2ns_scale(unsigned long khz, int cpu, > + unsigned long long tsc_now, > + unsigned long long sched_now) sched_now is not a real good name for this as it's only used at initialization time. So the argument name should reflect this otherwise you wonder yourself when looking at that code 6 month from now, why it's 0 on all the run time call sites. init_offset or some other sensible name which makes the purpose entirely clear. > void __init tsc_init(void) > { > - u64 lpj, cyc; > + u64 lpj, cyc, sch; sch? what's wrong with sched_now or now? It's not that there is a 3 letter limit. > int cpu; > > if (!boot_cpu_has(X86_FEATURE_TSC)) { > @@ -1403,9 +1405,10 @@ void __init tsc_init(void) > * up if their speed diverges) > */ > cyc = rdtsc(); > + sch = local_clock(); > for_each_possible_cpu(cpu) { > cyc2ns_init(cpu); > - set_cyc2ns_scale(tsc_khz, cpu, cyc); > + set_cyc2ns_scale(tsc_khz, cpu, cyc, sch); > } Thanks, tglx