From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933907AbXK3AAx (ORCPT ); Thu, 29 Nov 2007 19:00:53 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757918AbXK3AAk (ORCPT ); Thu, 29 Nov 2007 19:00:40 -0500 Received: from smtp2.linux-foundation.org ([207.189.120.14]:52064 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753733AbXK3AAj (ORCPT ); Thu, 29 Nov 2007 19:00:39 -0500 Date: Thu, 29 Nov 2007 15:59:40 -0800 From: Andrew Morton To: "Metzger, Markus T" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, hpa@zytor.com, tglx@linutronix.de, ak@suse.de, suresh.b.siddha@intel.com, markus.t.metzger@intel.com, Michael Kerrisk Subject: Re: [patch 0/2] x86, ptrace: support for branch trace store(BTS) Message-Id: <20071129155940.7df70ac2.akpm@linux-foundation.org> In-Reply-To: <029E5BE7F699594398CA44E3DDF5544401024075@swsmsx413.ger.corp.intel.com> References: <029E5BE7F699594398CA44E3DDF5544401024075@swsmsx413.ger.corp.intel.com> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 29 Nov 2007 08:14:10 -0000 "Metzger, Markus T" wrote: > Support for Intel's last branch recording to ptrace. This gives > debuggers > access to this hardware feature and allows them to show an execution > trace > of the debugged application. > > Last branch recording (see section 18.5 in the Intel 64 and IA-32 > Architectures Software Developer's Manual) allows taking an execution > trace of the running application without instrumentation. When a branch > is executed, the hardware logs the source and destination address in a > cyclic buffer given to it by the OS. > > This can be a great debugging aid. It shows you how exactly you got > where you currently are without requiring you to do lots of single > stepping and rerunning. > > This patch manages the various buffers, configures the trace > hardware, disentangles the trace, and provides a user interface via > ptrace. On the high-level design: > - there is one optional trace buffer per thread_struct > - upon a context switch, the trace hardware is reconfigured to either > disable tracing or to use the appropriate buffer for the new task. > - tracing induces ~20% overhead as branch records are sent out on > the bus. > - the hardware collects trace per processor. To disentangle the > traces for different tasks, we use separate buffers and reconfigure > the trace hardware. > - the low-level data layout is configured at cpu initialization time > - different processors use different branch record formats > > > patch 1/2 contains the kernel changes > patch 2/2 contains changes to the ptrace man pages > > Is there any userspace code avaialble which people can use to play with this? How do you envisage it being used in the long term? Do you expect any of the standard performance tuning tools will be tweaked to understand this feature and if so which ones? I'm generally wondering "how will developers be using this in a year or two's time?" Please cc Michael Kerrisk on future versions of these patches. The patches were horridly wordwrapped. Is there any likelihood that any other CPUs do now or will in the future support any similar feature to this? If so, is an implementation which is 100% contained to arch/x86 appropriate?