Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761350AbZDQArf (ORCPT ); Thu, 16 Apr 2009 20:47:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760829AbZDQAnr (ORCPT ); Thu, 16 Apr 2009 20:43:47 -0400 Received: from gw.goop.org ([64.81.55.164]:44452 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760035AbZDQAnq (ORCPT ); Thu, 16 Apr 2009 20:43:46 -0400 Message-ID: <49E7D0BC.4070700@goop.org> Date: Thu, 16 Apr 2009 17:43:40 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Mathieu Desnoyers CC: Steven Rostedt , linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton , Thomas Gleixner , Peter Zijlstra , Frederic Weisbecker , Theodore Tso , Arjan van de Ven , Christoph Hellwig , Lai Jiangshan , Zhaolei , Li Zefan , KOSAKI Motohiro , Masami Hiramatsu , "Frank Ch. Eigler" , Tom Zanussi , Jiaying Zhang , Michael Rubin , Martin Bligh , Peter Zijlstra , Neil Horman , Eduard - Gabriel Munteanu , Pekka Enberg Subject: Re: [PATCH 2/8] tracing: create automated trace defines References: <20090414172640.796858018@goodmis.org> <49E51FC1.8090306@goop.org> <20090415014548.GA7984@Krystal> <49E6065B.7080409@goop.org> <20090416023456.GC22378@Krystal> <49E69E76.9030608@goop.org> <20090416234410.GA20513@Krystal> <49E7C74A.8030100@goop.org> <20090417001317.GB20513@Krystal> <49E7CADE.1060202@goop.org> <20090417002831.GC20513@Krystal> In-Reply-To: <20090417002831.GC20513@Krystal> X-Enigmail-Version: 0.95.6 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2912 Lines: 58 Mathieu Desnoyers wrote: > "all this code" is actually : > > rcu_read_lock_sched_notrace(); \ > it_func = rcu_dereference((tp)->funcs); \ > if (it_func) { \ > do { \ > ((void(*)(proto))(*it_func))(args); \ > } while (*(++it_func)); \ > } \ > rcu_read_unlock_sched_notrace(); \ > > Which does nothing more than disabling preemption and a for loop to > call all the tracepoint handlers. I don't see the big win in laying out > the stack to call this code out-of-line; we would just remove the > preempt disable and the loop, which are minimal compared to most > call stacks. > Well, look at it from my perspective: Ingo has been repeatedly beating me up for the overhead pvops adds to a native kernel, where it really is just a (direct) function call. I want to instrument each pvop site with a tracepoint so I can actually work out which calls are being called how frequently to look for new optimisation opportunities. I would guess the tracepoint code sequence is going to increase the impact of each pvop call site by a fair bit, and that's not counting the effects the extra register pressure will have. That's a pile of code to add. And frankly, that's fine by me, because I would expect this degree of introspection to have some performance hit. But it does make the need for per-subsystem tracing Kconfig entries fairly important, because I don't think this would be acceptable to ship in a non-debug-everything kernel build, even though other tracepoints might be. > So basically, tracepoints are already just doing a function call, with a > few more bytes for preempt disable and multiple handler support. > > About the compiler deciding to put the unlikely branch out-of-line, I've > never seen any function calls generated just for the sake of saving > those few bytes, that would be crazy of the part of the compiler. > However, it can (and should) freely put the stack setup in the coldest > cache-lines possible, which are reachable by a near jump. > No, it wouldn't generate a call. But if its going to put the code out of line into cold cache-lines, then it may as well generate a call. Anyway, the important point from my perspective is that tracepoint.h have no #include dependencies beyond linux/types.h (compiler.h, etc). J -- 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/