From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with SMTP id 6BA039000BD for ; Fri, 23 Sep 2011 07:54:20 -0400 (EDT) Message-ID: <4E7C7353.50802@hitachi.com> Date: Fri, 23 Sep 2011 20:53:55 +0900 From: Masami Hiramatsu MIME-Version: 1.0 Subject: Re: [PATCH v5 3.1.0-rc4-tip 8/26] x86: analyze instruction and determine fixups. References: <20110920115938.25326.93059.sendpatchset@srdronam.in.ibm.com> <20110920120127.25326.71509.sendpatchset@srdronam.in.ibm.com> <20110920171310.GC27959@stefanha-thinkpad.localdomain> <20110920181225.GA5149@infradead.org> <20110920205317.GA1508@stefanha-thinkpad.localdomain> In-Reply-To: <20110920205317.GA1508@stefanha-thinkpad.localdomain> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Stefan Hajnoczi Cc: Christoph Hellwig , Srikar Dronamraju , Peter Zijlstra , Ingo Molnar , Steven Rostedt , Linux-mm , Arnaldo Carvalho de Melo , Linus Torvalds , Hugh Dickins , Andi Kleen , Thomas Gleixner , Jonathan Corbet , Oleg Nesterov , Andrew Morton , Jim Keniston , Roland McGrath , Ananth N Mavinakayanahalli , LKML (2011/09/21 5:53), Stefan Hajnoczi wrote: > On Tue, Sep 20, 2011 at 02:12:25PM -0400, Christoph Hellwig wrote: >> On Tue, Sep 20, 2011 at 06:13:10PM +0100, Stefan Hajnoczi wrote: >>> You've probably thought of this but it would be nice to skip XOL for >>> nops. This would be a common case with static probes (e.g. sdt.h) where >>> the probe template includes a nop where we can easily plant int $0x3. >> >> Do we now have sdt.h support for uprobes? That's one of the killer >> features that always seemed to get postponed. > > Not yet but it's a question of doing roughly what SystemTap does to > parse the appropriate ELF sections and then putting those probes into > uprobes. > > Masami looked at this and found that SystemTap sdt.h currently requires > an extra userspace memory store in order to activate probes. Each probe > has a "semaphore" 16-bit counter which applications may test before > hitting the probe itself. This is used to avoid overhead in > applications that do expensive argument processing (e.g. creating > strings) for probes. Indeed, originally, those semaphores designed for such use cases. However, some applications *always* use it (e.g. qemu-kvm). > > But this should be solvable so it would be possible to use perf-probe(1) > on a std.h-enabled binary. Some distros already ship such binaries! I'm not sure that we should stick on the current implementation of the sdt.h. I think we'd better modify the sdt.h to replace such semaphores with checking whether the tracepoint is changed from nop. Or, we can introduce an add-hoc ptrace code to perftools for modifying those semaphores. However, this means that user always has to use perf to trace applications, and it's hard to trace multiple applications at a time (can we attach all of them?)... Thank you, -- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu.pt@hitachi.com -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org