Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754585Ab3HES3D (ORCPT ); Mon, 5 Aug 2013 14:29:03 -0400 Received: from mga01.intel.com ([192.55.52.88]:5610 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753699Ab3HES3C (ORCPT ); Mon, 5 Aug 2013 14:29:02 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.89,820,1367996400"; d="scan'208";a="376507597" Message-ID: <51FFEEEC.5060902@linux.intel.com> Date: Mon, 05 Aug 2013 11:29:00 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Steven Rostedt CC: Linus Torvalds , LKML , gcc , Ingo Molnar , Mathieu Desnoyers , Thomas Gleixner , David Daney , Behan Webster , Peter Zijlstra , Herbert Xu Subject: Re: [RFC] gcc feature request: Moving blocks into sections References: <1375721715.22073.80.camel@gandalf.local.home> <1375725328.22073.101.camel@gandalf.local.home> <51FFEC56.6040206@linux.intel.com> <1375727010.22073.110.camel@gandalf.local.home> In-Reply-To: <1375727010.22073.110.camel@gandalf.local.home> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1621 Lines: 38 On 08/05/2013 11:23 AM, Steven Rostedt wrote: > On Mon, 2013-08-05 at 11:17 -0700, H. Peter Anvin wrote: >> On 08/05/2013 10:55 AM, Steven Rostedt wrote: >>> >>> Well, as tracepoints are being added quite a bit in Linux, my concern is >>> with the inlined functions that they bring. With jump labels they are >>> disabled in a very unlikely way (the static_key_false() is a nop to skip >>> the code, and is dynamically enabled to a jump). >>> >> >> Have you considered using traps for tracepoints? A trapping instruction >> can be as small as a single byte. The downside, of course, is that it >> is extremely suppressed -- the trap is always expensive -- and you then >> have to do a lookup to find the target based on the originating IP. > > No, never considered it, nor would I. Those that use tracepoints, do use > them extensively, and adding traps like this would probably cause > heissenbugs and make tracepoints useless. > > Not to mention, how would we add a tracepoint to a trap handler? > Traps nest, that's why there is a stack. (OK, so you don't want to take the same trap inside the trap handler, but that code should be very limited.) The trap instruction just becomes very short, but rather slow, call-return. However, when you consider the cost you have to consider that the tracepoint is doing other work, so it may very well amortize out. -hpa -- 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/