From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [patch 4/4] KVM-trace port to tracepoints Date: Wed, 23 Jul 2008 09:20:24 -0400 Message-ID: <20080723132024.GC15401@Krystal> References: <20080717155724.897537670@polymtl.ca> <20080717160003.359557938@polymtl.ca> <487F7800.4010502@siemens.com> <20080717172853.GB29855@Krystal> <488604F8.1040008@siemens.com> <48862B01.7070907@qumranet.com> <1216799346.7257.125.camel@twins> <4886E6FD.4090200@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: Peter Zijlstra , Jan Kiszka , akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, "Feng(Eric) Liu" To: Avi Kivity Return-path: Received: from tomts16.bellnexxia.net ([209.226.175.4]:59481 "EHLO tomts16-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750996AbYGWNU1 (ORCPT ); Wed, 23 Jul 2008 09:20:27 -0400 Content-Disposition: inline In-Reply-To: <4886E6FD.4090200@qumranet.com> Sender: kvm-owner@vger.kernel.org List-ID: * Avi Kivity (avi@qumranet.com) wrote: > Peter Zijlstra wrote: >> On Tue, 2008-07-22 at 21:46 +0300, Avi Kivity wrote: >> >>> Jan Kiszka wrote: >>> >>>> That's true - as long as you don't have to add/remove/modify >>>> tracepoints. I had to do this job in the past (not for KVM). Having 1 >>>> spot in 1 file (based on generic probes) would be handier in that case >>>> than 5 spots in 3 files. But if the KVM tracepoints are considered >>>> stable in their number and structure, that shouldn't be an issue here. >>>> >>>> >>> Tracepoints aren't stable; they are artefacts of the implementation. >>> >> >> Which IMHO makes it unsuitable for trace_mark() as that will be exported >> to user-space, and every time you change your tracepoints you'll change >> user visible things - not nice. >> > > They are used for debugging (mostly performance related), so changes are > expected. > > What uses of trace_mark() depend on a stable interface? blktrace? > Actually, LTTng likes to have the { marker name, field name } pairs unchanged for the markers it looks for, but that's about it. If a userspace analysis plugin fails to see a marker (because it is disabled or changed), it just does not apply its particular analysis on the trace. Since the markers and marker types are self-described in the trace, userspace can detect any change the the present markers, so there is no need to rely on "version numbers" because we are able to proceed to a complete marker list verification (names, field names, types) before starting the trace analysis. Mathieu -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68