From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756181Ab0JSW1b (ORCPT ); Tue, 19 Oct 2010 18:27:31 -0400 Received: from terminus.zytor.com ([198.137.202.10]:35313 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754085Ab0JSW1a (ORCPT ); Tue, 19 Oct 2010 18:27:30 -0400 Message-ID: <4CBE1B04.5020309@zytor.com> Date: Tue, 19 Oct 2010 15:26:12 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc13 Thunderbird/3.1.4 MIME-Version: 1.0 To: Steven Rostedt CC: Thomas Gleixner , Mathieu Desnoyers , Koki Sanagi , Peter Zijlstra , Ingo Molnar , Frederic Weisbecker , nhorman@tuxdriver.com, scott.a.mcmillan@intel.com, laijs@cn.fujitsu.com, LKML , eric.dumazet@gmail.com, kaneshige.kenji@jp.fujitsu.com, David Miller , izumi.taku@jp.fujitsu.com, kosaki.motohiro@jp.fujitsu.com, Heiko Carstens , "Luck, Tony" Subject: Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints References: <20100908112529.GA25931@elte.hu> <1287395077.29097.1543.camel@twins> <1287398936.29097.1548.camel@twins> <4CBD79CF.2060706@jp.fujitsu.com> <20101019132236.GA19197@Krystal> <1287496495.16971.372.camel@gandalf.stny.rr.com> <20101019142820.GA14520@Krystal> <1287521757.16971.397.camel@gandalf.stny.rr.com> <1287523439.16971.433.camel@gandalf.stny.rr.com> <4CBE122B.9020807@zytor.com> <1287527005.16971.527.camel@gandalf.stny.rr.com> In-Reply-To: <1287527005.16971.527.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/19/2010 03:23 PM, Steven Rostedt wrote: > On Tue, 2010-10-19 at 14:48 -0700, H. Peter Anvin wrote: >> On 10/19/2010 02:23 PM, Steven Rostedt wrote: >>> >>> But it seemed that gcc for you inlined the code in the wrong spot. >>> Perhaps it's not a good idea to have the something like h - softirq_vec >>> in the parameter of the tracepoint. Not saying that your change is not >>> worth it. It is, because h - softirq_vec is used by others now too. >>> >> >> OK, first of all, there are some serious WTFs here: >> >> # define JUMP_LABEL_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t" >> >> A jump instruction is one of the worst possible NOPs. Why are we doing >> this? > > Good question. Safety? Jason? > > This is the initial jumps and are converted on boot up to a better nop. > But it makes absolutely no sense to insert an instruction that suboptimal and then convert it. Start out with a reasonable, universally acceptable, instruction, e.g. LEA on 32 bits and NOPL on 64 bits. >> >> The second thing that I found when implementing static_cpu_has() was >> that it is actually better to encapsulate the asm goto in a small inline >> which returns bool (true/false) -- gcc will happily optimize out the >> variable and only see it as a flow of control thing. I would be very >> curious if that wouldn't make gcc generate better code in cases like that. >> >> gcc 4.5.0 has a bug in that there must be a flowthrough case in the asm >> goto (you can't have it unconditionally branch one way or the other), so >> that should be the likely case and accordingly it should be annotated >> likely() so that gcc doesn't reorder. I suspect in the end one ends up >> with code like this: >> >> static __always_inline __pure bool __switch_point(...) >> { >> asm goto("1: " JUMP_LABEL_INITIAL_NOP >> /* ... patching stuff */ >> : : : : t_jump); >> return false; >> t_jump: >> return true; >> } >> >> #define SWITCH_POINT(x) unlikely(__switch_point(x)) >> >> I *suspect* this will resolve the need for hot/cold labels just fine. > > Interesting, we could try this. > It of course also have the nice property that it syntactically looks exactly like any other C conditional. -hpa