From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from newverein.lst.de (verein.lst.de [213.95.11.211]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 565AC1A000C for ; Tue, 16 Feb 2016 21:35:55 +1100 (AEDT) Date: Tue, 16 Feb 2016 11:35:51 +0100 From: Torsten Duwe To: Michael Ellerman Cc: Balbir Singh , Jiri Kosina , Miroslav Benes , Petr Mladek , Jessica Yu , Steven Rostedt , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org Subject: Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Message-ID: <20160216103551.GA25103@lst.de> References: <20160210174221.EBBEC692C8@newverein.lst.de> <20160210174450.9C065692C8@newverein.lst.de> <1455176897.2885.12.camel@gmail.com> <20160211084230.GB29683@lst.de> <1455532035.26205.5.camel@ellerman.id.au> <20160215140408.GA22872@lst.de> <1455617356.31947.2.camel@ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1455617356.31947.2.camel@ellerman.id.au> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Feb 16, 2016 at 09:09:16PM +1100, Michael Ellerman wrote: > On Mon, 2016-02-15 at 15:04 +0100, Torsten Duwe wrote: > > If you use "-pg -mprofile-kernel", gcc seems to forget that, and omits the TOC > > load, for a similar assembler calling sequence. > > That's by design. Ah, ok. > mprofile-kernel is supposed to create as little overhead as possible in the > non-traced case. All of the burden is shifted to the trace function (_mcount). ... or its helpers, see below. > The reason to do that is because modern distros always build with tracing, but > most of the time tracing will not actually be active. So we want the cost of > tracing-built-in-but-disabled to be ~zero. Ok, that's a design goal. > > That was the alternative I asked about; but given that the _mcount / ftrace_caller > > trampoline hardly differs from a normal trampoline (so far), loading R2 would be the > > general case, or an excessive special case handling would result. > > I'm not sure I follow what you mean there at the end. This suggests you have not yet actively debugged this problem ;-) > Requiring ftrace_caller() to load the kernel TOC is not a problem IMHO. The problem is, you don't get to ftrace_caller in the first place :) > I think I have an easier way to do it, I'll reply to the patch with that (if it > works). I doubt so. Either it works, _or_ it is easier ;) To save you some work: by the design of minimal overhead you try to follow, SQUASH_TOC_SAVE_INSN from my patches isn't sufficient. You'll need to load the _current_ TOC _on_ the trampoline, and in turn it will be different from the regular trampolines; and that needs to be recognised, or the normal module linker logic won't work. OTOH my proposed GCC change only affects a very limited number of functions... Looking forward to your patch! Torsten