From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755087Ab0EGPIl (ORCPT ); Fri, 7 May 2010 11:08:41 -0400 Received: from mail.openrapids.net ([64.15.138.104]:54961 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754707Ab0EGPIk (ORCPT ); Fri, 7 May 2010 11:08:40 -0400 Date: Fri, 7 May 2010 11:08:38 -0400 From: Mathieu Desnoyers To: Steven Rostedt Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton , Thomas Gleixner , Peter Zijlstra , Arnaldo Carvalho de Melo , Lai Jiangshan , Li Zefan , Masami Hiramatsu , Christoph Hellwig Subject: Re: [PATCH 2/9 - v2][RFC] tracing: Let tracepoints have data?passed to tracepoint callbacks Message-ID: <20100507150838.GB30356@Krystal> References: <7a22ce9b-4c74-4818-9521-7fbccd1b8b1d@email.android.com> <20100507143901.GA18408@Krystal> <1273244123.22438.140.camel@gandalf.stny.rr.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1273244123.22438.140.camel@gandalf.stny.rr.com> X-Editor: vi X-Info: http://www.efficios.com X-Operating-System: Linux/2.6.26-2-686 (i686) X-Uptime: 11:04:30 up 104 days, 17:41, 8 users, load average: 0.14, 0.16, 0.11 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Steven Rostedt (rostedt@goodmis.org) wrote: > On Fri, 2010-05-07 at 10:39 -0400, Mathieu Desnoyers wrote: > > * Steven Rostedt (rostedt@goodmis.org) wrote: > > > > > > > > > "Frederic Weisbecker" wrote: > > > > > > >On Mon, May 03, 2010 at 11:40:47PM -0400, Steven Rostedt wrote: > > [...] > > > >> > > > >> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h > > > >> index 78b4bd3..ee8059a 100644 > > > >> --- a/include/linux/tracepoint.h > > > >> +++ b/include/linux/tracepoint.h > > > >> @@ -20,12 +20,17 @@ > > > >> struct module; > > > >> struct tracepoint; > > > >> > > > >> +struct tracepoint_func { > > > >> + void *func; > > > >> + void *data; > > > >> +}; > > > >> + > > > >> struct tracepoint { > > > >> const char *name; /* Tracepoint name */ > > > >> int state; /* State. */ > > > >> void (*regfunc)(void); > > > >> void (*unregfunc)(void); > > > >> - void **funcs; > > > >> + struct tracepoint_func *funcs; > > > >> } __attribute__((aligned(32))); /* > > > >> * Aligned on 32 bytes because it is > > > >> * globally visible and gcc happily > > > >> @@ -46,14 +51,18 @@ struct tracepoint { > > > >> */ > > > >> #define __DO_TRACE(tp, proto, args) \ > > > >> do { \ > > > >> - void **it_func; \ > > > >> + struct tracepoint_func *it_func_ptr; \ > > > >> + void *it_func; \ > > > >> + void *__data; \ > > > >> \ > > > >> rcu_read_lock_sched_notrace(); \ > > > >> - it_func = rcu_dereference_sched((tp)->funcs); \ > > > >> - if (it_func) { \ > > > >> + it_func_ptr = rcu_dereference_sched((tp)->funcs); \ > > > >> + if (it_func_ptr) { \ > > > >> do { \ > > > >> - ((void(*)(proto))(*it_func))(args); \ > > > >> - } while (*(++it_func)); \ > > > >> + it_func = (it_func_ptr)->func; \ > > > >> + __data = (it_func_ptr)->data; \ > > > >> + ((void(*)(proto))(it_func))(args); \ > > > > > > > > > > > >So, we had a talk about this and we concluded that it is probably fine > > > >on every archs to push one more argument than needed in a function. > > > > > > > > > > Yeah, I'm hoping it's fine. > > > > How about changing the callback prototypes to match the call arguments (changing > > the type expected in register/unregister_trace, as well as an additional "check > > type" that I proposed for Ftrace) ? > > This can not happen!!!! As I said before, the register is done in C, > there is no macro that will help here. We create the call back with the > macro, but the registering is in kernel/trace/trace_events.c. One > register for ___ALL___ events!!! > > Thus there is no check. > > Understand this yet? Clearly understood. I was referring to add a static inline check_trace_##name##_callback_type(...) { } call within the callbacks you generate. It generates no code and adds compiler type-checking (rather than relying on CPP macro correctness for type-correctness). > > > > > > Otherwise, you basically expect here that: > > > > void fct(void *foo, void *bar, etc etc) (N parameters expected) > > { > > > > } > > > > called by: > > > > fct(foo, bar, etc etc, foobar) (N + 1 parameters) > > > > will always work. > > > > Can you show me where the C standard says it is safe to do so ? > > No, but it seems safe in the kernel ;-) The use of "seems" here does not give me a warm feeling of safety. ;) > > But that said. There is another option that will conform to this, and > that is to add flags to registering tracepoints. I already wrote a patch > for this in trying to do some other work (that I threw away). > > > So here's the proposal. > > Change struct tracepoint_func to... > > struct tracepoint_func { > void *func; > void *data; > unsigned int flags; > }; > > > The flags is set when registered. If a function is registered with data, > then the flags field will be set. Then the calling of the function can > be: > > if ((it_func_ptr)->flags & TP_FL_DATA) > ((void(*)(proto, void *))(it_func)(args, __data); > else > ((void(*)(proto))(it_func)(args); > > This would comply with the C standard. This would also add a branch on the tracing fast path, which I would like to avoid. Why can't we simply change all prototypes to take an extra void *__data parameter instead ? Thanks, Mathieu > > -- Steve > > -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com