From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757954Ab0JSVjE (ORCPT ); Tue, 19 Oct 2010 17:39:04 -0400 Received: from mail3.caviumnetworks.com ([12.108.191.235]:7018 "EHLO mail3.caviumnetworks.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753247Ab0JSVjC (ORCPT ); Tue, 19 Oct 2010 17:39:02 -0400 Message-ID: <4CBE0FEB.3070104@caviumnetworks.com> Date: Tue, 19 Oct 2010 14:38:51 -0700 From: David Daney User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.12) Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7 MIME-Version: 1.0 To: Jason Baron CC: Thomas Gleixner , Mathieu Desnoyers , Steven Rostedt , Koki Sanagi , Peter Zijlstra , Ingo Molnar , Frederic Weisbecker , nhorman@tuxdriver.com, scott.a.mcmillan@intel.com, laijs@cn.fujitsu.com, "H. Peter Anvin" , 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: <1287398936.29097.1548.camel@twins> <4CBD79CF.2060706@jp.fujitsu.com> <20101019132236.GA19197@Krystal> <1287496495.16971.372.camel@gandalf.stny.rr.com> <20101019142820.GA14520@Krystal> <4CBE0AC6.3040402@caviumnetworks.com> <20101019213225.GB2855@redhat.com> In-Reply-To: <20101019213225.GB2855@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 19 Oct 2010 21:39:17.0276 (UTC) FILETIME=[14CB8DC0:01CB6FD6] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/19/2010 02:32 PM, Jason Baron wrote: > On Tue, Oct 19, 2010 at 02:16:54PM -0700, David Daney wrote: >> On 10/19/2010 12:49 PM, Thomas Gleixner wrote: >> [...] >>> So that saves _TWO_ bytes of text and replaces: >>> >>> - 1e: 83 3d 00 00 00 00 00 cmpl $0x0,0x0(%rip) # 25 >>> - 25: 74 4d je 74 >>> + 1e: e9 00 00 00 00 jmpq 23 >>> + 23: eb 4d jmp 72 >>> >>> So it trades a conditional vs. two jumps ? WTF ?? >>> >>> I thought that jumplabel magic was supposed to get rid of the jump >>> over the tracing code ? In fact it adds another jump. Whatfor ? >> >> The 'asm goto' construct in GCC-4.5 is deficient in this area. >> >> GCC assumes that all exit paths from an 'asm goto' are equally >> likely, so the tracing (or dynamic printk etc.) code is assumed to >> be hot and is emitted inline. Since they are inline like this, >> there are all these jumps around them and they pollute the I-Cache. >> >> I was looking at fixing it, but I think a true general purpose fix >> would require enhancing GCC's grammar to allow specifying of the >> 'likelyness' of each exit path from 'asm goto'. >> >> David Daney >> > > right, the next step is adding support for hot/cold labels, so the > tracing code will be annotaed with a 'cold' label. Thus, not adding the > 'jmp' above on line '23', and in fact moving the tracing code > out-of-line. Maybe I haven't been clear on this. > Ok, so is anybody working on doing that? GCC-4.6 stage 1 (the time when a change like this could be merged) closes in 8 days. It is unfortunate that we have this shiny new feature that can't really be used because the infrastructure is only half baked. David Daney