Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758841AbYHHSlT (ORCPT ); Fri, 8 Aug 2008 14:41:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753206AbYHHSlK (ORCPT ); Fri, 8 Aug 2008 14:41:10 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.124]:42594 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752834AbYHHSlK (ORCPT ); Fri, 8 Aug 2008 14:41:10 -0400 Date: Fri, 8 Aug 2008 14:41:06 -0400 (EDT) From: Steven Rostedt X-X-Sender: rostedt@gandalf.stny.rr.com To: Mathieu Desnoyers cc: linux-kernel@vger.kernel.org, Ingo Molnar , Thomas Gleixner , Peter Zijlstra , Andrew Morton , Linus Torvalds , David Miller , Roland McGrath , Ulrich Drepper , Rusty Russell , Jeremy Fitzhardinge , Gregory Haskins , Arnaldo Carvalho de Melo , "Luis Claudio R. Goncalves" , Clark Williams Subject: Re: [PATCH 0/5] ftrace: to kill a daemon In-Reply-To: <20080808182104.GA11376@Krystal> Message-ID: References: <20080807182013.984175558@goodmis.org> <20080807184741.GB18164@Krystal> <20080808172259.GB8244@Krystal> <20080808174607.GG8244@Krystal> <20080808182104.GA11376@Krystal> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) 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: 1912 Lines: 54 On Fri, 8 Aug 2008, Mathieu Desnoyers wrote: > * Steven Rostedt (rostedt@goodmis.org) wrote: > > > > > > > > > > That's bad : > > > > > > #define GENERIC_NOP5 GENERIC_NOP1 GENERIC_NOP4 > > > > > > #define K8_NOP5 K8_NOP3 K8_NOP2 > > > > > > #define K7_NOP5 K7_NOP4 ASM_NOP1 > > > > > > So, when you try, later, to replace these instructions with a single > > > 5-bytes instruction, a preempted thread could iret in the middle of your > > > 5-bytes insn and cause an illegal instruction ? > > > > That's why I use kstop_machine. > > > > kstop_machine does not guarantee that you won't have _any_ thread > preempted with IP pointing exactly in the middle of your instructions > _before_ the modification scheduled back in _after_ the modification and > thus causing an illegal instruction. > > Still buggy. :/ Hmm, good point. Unless... Can a processor be preempted in a middle of nops? What do nops do for a processor? Can it skip them nicely in one shot? This means I'll have to do the benchmarks again, and see what the performance difference of a jmp and a nop is significant. I'm thinking that if the processor can safely skip nops without any type of processing, this may be the reason that nops are better than a jmp. A jmp causes the processor to do a little more work. I might even run a test to see if I can force a processor that uses the three-two nops to preempt between them. I can add a test in x86 ftrace.c to check to see which nop was used, and use the jmp if the arch does not have a 5 byte nop. I'm assuming that jmp is more expensive than the nops because otherwise a jmp 0 would have been used as a 5 byte nop. -- Steve -- 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/