From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756179AbZEUCPf (ORCPT ); Wed, 20 May 2009 22:15:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754070AbZEUCP1 (ORCPT ); Wed, 20 May 2009 22:15:27 -0400 Received: from smtp-out.google.com ([216.239.33.17]:12566 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753958AbZEUCP1 (ORCPT ); Wed, 20 May 2009 22:15:27 -0400 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=mime-version:in-reply-to:references:date:message-id:subject:from:to: cc:content-type:content-transfer-encoding:x-system-of-record; b=nwr9ZYL2YDA37VeTLQ0FEPePrAGTCgs9EjiO5g0KPn9Dr+RsxmTGaOJWaEA8VS9N7 8iFHMnABKr17h36GUGLVg== MIME-Version: 1.0 In-Reply-To: <4A14B56F.6030109@cn.fujitsu.com> References: <20090520073348.GA12316@elte.hu> <20090520154241.GC3081@redhat.com> <5df78e1d0905201849l5e71953bg8444166ae3c8f92a@mail.gmail.com> <4A14B56F.6030109@cn.fujitsu.com> Date: Wed, 20 May 2009 19:15:23 -0700 Message-ID: <5df78e1d0905201915l6b81ef7cn8d1836faf9e11726@mail.gmail.com> Subject: Re: [PATCH 0/3] tracepoints: delay argument evaluation From: Jiaying Zhang To: Li Zefan Cc: Jason Baron , Ingo Molnar , linux-kernel@vger.kernel.org, fweisbec@gmail.com, laijs@cn.fujitsu.com, rostedt@goodmis.org, peterz@infradead.org, mathieu.desnoyers@polymtl.ca, mbligh@google.com, roland@redhat.com, fche@redhat.com Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org But if we convert blktrace to use event tracer interface, we can have: trace_block_bio_complete(md, bio); TRACE_EVENT(block_bio_complete, TP_PROTO(struct mapped_device *md, struct bio *bio), ... TP_fast_assign( __entry->queue = md->queue; ... ), ); Jiaying On Wed, May 20, 2009 at 6:59 PM, Li Zefan wrote: > Jiaying Zhang wrote: >> Is it possible to convert blktrace to use event tracer? Then in this case we > > Yes, I'm doing this, see: > http://marc.info/?l=linux-kernel&m=124228198011297&w=2 > >> can pass 'md' as the parameter to trace_block_bio_complete and dereference >> md->queue during assignment. >> > > But the problem discussed here exists whether you use plain tracepoints > or TRACE_EVENT. > > Though we can add a new tracepoint named trace_md_bio_complete, this is > not the way to solve it. > >> Jiaying >> >> On Wed, May 20, 2009 at 8:42 AM, Jason Baron wrote: >>> On Wed, May 20, 2009 at 09:33:48AM +0200, Ingo Molnar wrote: >>>> hm, this is really a compiler bug in essence - the compiler should >>>> delay the construction of arguments into unlikely branches - if the >>>> arguments are only used there. >>>> >>>> We'd basically open-code a clear-cut: >>>> >>>> trace_block_bio_complete(md->queue, bio); >>>> >>>> into this form: >>>> >>>> trace(block_bio_complete, md->queue, bio); >>>> >>>> .. and this latter form could become moot (and a nuisance) if the >>>> compiler is fixed. >>>> >>>> Have you tried very latest GCC, does it still have this optimization >>>> problem? >>>> >>>> Note that the compiler getting this right would help a _lot_ of >>>> other inline functions in the kernel as well. Arguments only used >>>> within unlikely() branches are quite common. >>>> >>>> Ingo >>> hi, >>> >>> I e-mailed the gcc list, where they suggested using a macro, as I've >>> done. They also suggested filing an enhancement request for this, which >>> I've done: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40207 It seems >>> like they agree with the suggestion. >>> >>> It still might make sense to make this requirement explicit (by adding >>> the extra macro), as the tracepoint off case should really be as optimized as >>> possible. >>> >>> thanks, >>> >>> -Jason >>> >