Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752268AbaLSRDw (ORCPT ); Fri, 19 Dec 2014 12:03:52 -0500 Received: from casper.infradead.org ([85.118.1.10]:45475 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751218AbaLSRDv (ORCPT ); Fri, 19 Dec 2014 12:03:51 -0500 Date: Fri, 19 Dec 2014 18:03:34 +0100 From: Peter Zijlstra To: Andy Lutomirski Cc: Shaohua Li , "linux-kernel@vger.kernel.org" , X86 ML , Kernel-team@fb.com, "H. Peter Anvin" , Ingo Molnar , John Stultz Subject: Re: [PATCH v2 3/3] X86: Add a thread cpu time implementation to vDSO Message-ID: <20141219170334.GM30905@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 19, 2014 at 08:48:07AM -0800, Andy Lutomirski wrote: > 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? Yeah, I think you should be able to use that. You could count a NOP event and simply use its activated time. We have PERF_COUNT_SW_DUMMY for such purposes iirc. The advantage of using perf self profiling is that it (obviously) extends to more than just walltime. -- 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/