From mboxrd@z Thu Jan 1 00:00:00 1970 From: fche@redhat.com (Frank Ch. Eigler) Subject: Re: [patch 0/4] Port KVM-trace to tracepoints Date: Tue, 22 Jul 2008 15:16:12 -0400 Message-ID: References: <20080717155724.897537670@polymtl.ca> <48862A30.7050701@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Mathieu Desnoyers , akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org, Peter Zijlstra , kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from mx1.redhat.com ([66.187.233.31]:53663 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751061AbYGVTTb (ORCPT ); Tue, 22 Jul 2008 15:19:31 -0400 In-Reply-To: <48862A30.7050701@qumranet.com> (Avi Kivity's message of "Tue, 22 Jul 2008 21:42:56 +0300") Sender: kvm-owner@vger.kernel.org List-ID: Avi Kivity writes: > [...] > kvm tracepoints are heavily tied into the implementation; and making > them harder to write means we will have less information. In fact, I > am contemplating moving in another direction (when looking at the > pgprintk()s scattered around arch/x86/kvm/mmu.c: > > kvm_trace("pfentry", "page_fault entry addr %lx error code %x\n", > cr2, error_code); > > Unlike printk()s, no actual formatting would occur during runtime. Have you considered using trace_mark() directly - eliminating the KVM_TRACEN() middlemen? > Instead, at initialization time all the strings would be parsed into > a data structure that describes the data types, and the runtime > would simply consult this structure and copy the arguments into > trace records. User space would also be able to pull this structure > and so recreate the formatted string. If one really wanted to, one could build such a mechanism on top of marker-based callbacks. > The advantages I see to this are: > > - easy to add traces; the most important advantage > - when the code changes, obsolete traces are completely removed > - good performance Ditto. > - no need to have a formats file in userspace (which is tied to the > kernel version) OTOH, you'd have the kernel collecting compact binary records containing just the parameters, which are at least as tied to kernel version. > - can also send printk()s along, for synchronization with other kvm > and kernel events Ditto. It is elementary to attach a printk-generating marker callback. - FChE