From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755032Ab3BWGEi (ORCPT ); Sat, 23 Feb 2013 01:04:38 -0500 Received: from mail-da0-f48.google.com ([209.85.210.48]:62551 "EHLO mail-da0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754870Ab3BWGEh (ORCPT ); Sat, 23 Feb 2013 01:04:37 -0500 Message-ID: <51285BF1.2090208@linaro.org> Date: Fri, 22 Feb 2013 22:04:33 -0800 From: John Stultz User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130106 Thunderbird/17.0.2 MIME-Version: 1.0 To: Peter Zijlstra CC: Thomas Gleixner , Stephane Eranian , Pawel Moll , LKML , "mingo@elte.hu" , Paul Mackerras , Anton Blanchard , Will Deacon , "ak@linux.intel.com" , Pekka Enberg , Steven Rostedt , Robert Richter Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples References: <1350408232.2336.42.camel@laptop> <1359728280.8360.15.camel@hornet> <51118797.9080800@linaro.org> <5123C3AF.8060100@linaro.org> <1361356160.10155.22.camel@laptop> In-Reply-To: <1361356160.10155.22.camel@laptop> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/20/2013 02:29 AM, Peter Zijlstra wrote: > On Tue, 2013-02-19 at 10:25 -0800, John Stultz wrote: >> So describe how the perf time domain is different then >> CLOCK_MONOTONIC_RAW. > The primary difference is that the trace/sched/perf time domain is not > strictly monotonic, it is only locally monotonic -- that is two time > stamps taken on the same cpu are guaranteed to be monotonic. So how would a clock_gettime(CLOCK_PERF,...) interface help you figure out which cpu you got your timestamp from? > Furthermore, to make it useful, there's an actual bound on the inter-cpu > drift (implemented by limiting the drift to CLOCK_MONOTONIC). So this sounds like you're already sort of interpolating to CLOCK_MONOTONIC, or am I just misunderstanding you? > Additionally -- to increase use -- we also added a monotonic sync point > when cpu A queries time of cpu B. Not sure I'm following this bit. But I'll have to go look at the code on Monday. > >> My concern here is that we're basically creating a kernel interface >> that >> exports implementation-defined semantics (again: whatever perf does >> right now). And I think folks want to do this, because adding >> CLOCK_PERF >> is easier then trying to: >> >> 1) Get a lock-free method for accessing CLOCK_MONOTONIC_RAW >> >> 2) Having perf interpolate its timestamps to CLOCK_MONOTONIC, or >> CLOCKMONOTONIC_RAW when it exports the data > Mostly cheaper, not easier. Given unstable TSC, MONOTONIC will have to > fall back to another clock source (hpet, acpi_pm and other assorted > crap). > > In order to avoid this, we'd had to relax the requirements. Using > anything other than TSC is simply not an option. Right, and this I understand. We can can play a little fast and lose with the rules for in-kernel uses, given the variety of hardware and the fact that performance is more critical then perfect accuracy. Since we're in-kernel we also have more information then userland does about what cpu we're running on, so we can get away with only locally-monotonic timestamps. But I want to be careful if we're exporting this out to userland that its both useful and that there's an actual specification for how CLOCK_PERF behaves, applications can rely upon not changing in the future. thanks -john