Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753092AbbEHQF0 (ORCPT ); Fri, 8 May 2015 12:05:26 -0400 Received: from mail-ig0-f181.google.com ([209.85.213.181]:36042 "EHLO mail-ig0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752045AbbEHQFZ (ORCPT ); Fri, 8 May 2015 12:05:25 -0400 MIME-Version: 1.0 In-Reply-To: <20150508104120.3de866e0@gandalf.local.home> References: <1430750469-16428-1-git-send-email-drew.richardson@arm.com> <199091314.42407.1430752205090.JavaMail.zimbra@efficios.com> <20150504200515.GA24626@dreric01-Precision-T1650> <20150504205748.GB21418@twins.programming.kicks-ass.net> <20150505145438.GA5145@dreric01-Precision-T1650> <20150508100903.39641009@gandalf.local.home> <20150508143037.GB1276@dreric01-Precision-T1650> <20150508104120.3de866e0@gandalf.local.home> Date: Fri, 8 May 2015 09:05:24 -0700 Message-ID: Subject: Re: [PATCHv3] ftrace: Provide trace clock monotonic raw From: John Stultz To: Steven Rostedt Cc: Drew Richardson , Peter Zijlstra , Mathieu Desnoyers , Ingo Molnar , "linux-kernel@vger.kernel.org" , Thomas Gleixner , Wade Cherry , Pawel Moll Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2043 Lines: 45 On Fri, May 8, 2015 at 7:41 AM, Steven Rostedt wrote: > On Fri, 8 May 2015 07:30:39 -0700 > Drew Richardson wrote: > >> Expose the NMI safe accessor to the monotonic raw clock to the >> tracer. The mono clock was added with commit >> 1b3e5c0936046e7e023149ddc8946d21c2ea20eb. The advantage of the >> monotonic raw clock is that it will advance more constantly than the >> monotonic clock. >> >> Imagine someone is trying to optimize a particular program to reduce >> instructions executed for a given workload while minimizing the effect >> on runtime. Also suppose that NTP is running and potentially making >> larger adjustments to the monotonic clock. If NTP is adjusting the >> monotonic clock to advance more rapidly, the program will appear to >> use fewer instructions per second but run longer than if the monotonic >> raw clock had been used. The total number of instructions observed >> would be the same regardless of the clock source used, but how it's >> attributed to time would be affected. >> >> Conversely if NTP is adjusting the monotonic clock to advance more >> slowly, the program will appear to use more instructions per second >> but run more quickly. Of course there are many sources that can cause >> jitter in performance measurements on modern processors, but let's >> remove NTP from the list. >> >> The monotonic raw clock can also be useful for tracing early boot, >> e.g. when debugging issues with NTP. >> > > Peter, Thomas, John, you OK with this? Yea. No objections from me wrt the functionality of the patch. I'd just again maybe caution folks to not go too far in assuming CLOCK_MONOTONIC_RAW doesn't have frequency changes, as the underlying hardware will drift due to varying thermal conditions. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/