Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751364AbaLSQsc (ORCPT ); Fri, 19 Dec 2014 11:48:32 -0500 Received: from mail-la0-f54.google.com ([209.85.215.54]:37994 "EHLO mail-la0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751456AbaLSQsa (ORCPT ); Fri, 19 Dec 2014 11:48:30 -0500 MIME-Version: 1.0 In-Reply-To: <20141219112350.GJ30905@twins.programming.kicks-ass.net> References: <8559794d3a1924408a811a2881ab916fffb6015b.1418857018.git.shli@fb.com> <95a7ba1a95a6251439d5ca2d3d56fe7f0778cb95.1418857018.git.shli@fb.com> <20141219112350.GJ30905@twins.programming.kicks-ass.net> From: Andy Lutomirski Date: Fri, 19 Dec 2014 08:48:07 -0800 Message-ID: Subject: Re: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO To: Peter Zijlstra Cc: Shaohua Li , "linux-kernel@vger.kernel.org" , X86 ML , Kernel-team@fb.com, "H. Peter Anvin" , Ingo Molnar , John Stultz Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 19, 2014 at 3:23 AM, Peter Zijlstra wrote: > On Thu, Dec 18, 2014 at 04:22:59PM -0800, Andy Lutomirski wrote: >> Bad news: this patch is incorrect, I think. Take a look at >> update_rq_clock -- it does fancy things involving irq time and >> paravirt steal time. So this patch could result in extremely >> non-monotonic results. > > Yeah, I'm not sure how (and if) we could make all that work :/ I obviously can't comment on what Facebook needs, but if I were rigging something up to profile my own code*, I'd want a count of elapsed time, including user, system, and probably interrupt as well. I would probably not want to count time during which I'm not scheduled, and I would also probably not want to count steal time. The latter makes any implementation kind of nasty. The API presumably doesn't need to be any particular clock id for clock_gettime, and it may not even need to be clock_gettime at all. Is perf self-monitoring good enough for this? If not, can we make it good enough? * I do this today using CLOCK_MONOTONIC --Andy -- Andy Lutomirski AMA Capital Management, LLC -- 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/