Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754478AbYJTULX (ORCPT ); Mon, 20 Oct 2008 16:11:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753388AbYJTULP (ORCPT ); Mon, 20 Oct 2008 16:11:15 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:43796 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753069AbYJTULO (ORCPT ); Mon, 20 Oct 2008 16:11:14 -0400 Date: Mon, 20 Oct 2008 13:10:02 -0700 (PDT) From: Linus Torvalds To: Mathieu Desnoyers cc: "Luck, Tony" , Steven Rostedt , Andrew Morton , Ingo Molnar , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" , Peter Zijlstra , Thomas Gleixner , David Miller , Ingo Molnar , "H. Peter Anvin" Subject: Re: [RFC patch 15/15] LTTng timestamp x86 In-Reply-To: <20081017184215.GB9874@Krystal> Message-ID: References: <20081016232729.699004293@polymtl.ca> <20081016234657.837704867@polymtl.ca> <20081017012835.GA30195@Krystal> <57C9024A16AD2D4C97DC78E552063EA3532D455F@orsmsx505.amr.corp.intel.com> <20081017172515.GA9639@goodmis.org> <57C9024A16AD2D4C97DC78E552063EA3533458AC@orsmsx505.amr.corp.intel.com> <20081017184215.GB9874@Krystal> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1934 Lines: 44 On Fri, 17 Oct 2008, Mathieu Desnoyers wrote: > > Hrm, on such systems > - *large* amount of cpus > - no synchronized TSCs > > What would be the best approach to order events ? My strong opinion has been - for a longish while now, and independently of any timestamping code - that we should be seriously looking at basically doing essentially a "ntp" inside the kernel to give up the whole idiotic notion of "synchronized TSCs". Yes, TSC's are often synchronized, but even when they are, we might as well _think_ of them as not being so. In other words, instead of expecting internal clocks to be synchronized, just make the clock be a clock network of independent TSC domains. The domains could in theory be per-package (assuming TSC is synchronized at that level), but even if we _could_ do that, we'd probably still be better off by simply always doing it per-core. If only because then the reading would be per-core. I think it's a mistake for us to maintain a single clock for gettimeofday() (well, "getnstimeofday" and the whole "clocksource_read()" crud to be technically correct). And sure, I bet clocksource_read() can do various per-CPU things and try to do that, but it's complex and pretty generic code, and as far as I know none of the clocksources have even tried. The TSC clocksource read certainly does not (it just does a very similar horrible "at least don't go backwards" crud that the LTTng patch suggested). So I think we should make "xtime" be a per-CPU thing, and add support for per-CPU clocksources. And screw that insane "mark_tsc_unstable()" thing. And if we did it well, we migth be able to get good timestamps that way too. Linus -- 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/