From mboxrd@z Thu Jan 1 00:00:00 1970 From: Namhyung Kim Subject: Re: [RFC/PATCH v2] ftrace: Reduce size of function graph entries Date: Wed, 29 Jun 2016 11:38:37 +0900 Message-ID: <20160629023837.GD1628@sejong> References: <1467091840-4625-1-git-send-email-namhyung@kernel.org> <20160628193200.71629fc5@gandalf.local.home> <20160629013234.GA1628@sejong> <20160628215738.6e4815dd@gandalf.local.home> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Return-path: Received: from LGEAMRELO12.lge.com ([156.147.23.52]:53203 "EHLO lgeamrelo12.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752449AbcF2Cim (ORCPT ); Tue, 28 Jun 2016 22:38:42 -0400 In-Reply-To: <20160628215738.6e4815dd@gandalf.local.home> Content-Disposition: inline Sender: linux-arch-owner@vger.kernel.org List-ID: To: Steven Rostedt Cc: Ingo Molnar , LKML , linux-arch@vger.kernel.org On Tue, Jun 28, 2016 at 09:57:38PM -0400, Steven Rostedt wrote: > On Wed, 29 Jun 2016 10:32:34 +0900 > Namhyung Kim wrote: > > > > is at 32-37 > > > > > > For a total of 38 bytes. I'm betting that without the packed, the 4 > > > extra bytes will always be at the end. > > > > Woundn't it be 36 or 40 bytes? :) > > Ug, don't know what I was counting then. I added the 64 bit version as > a after thought. > > > > > > > > > If the compiler places it incorrectly without any attribute, it will > > > fail to read the long long if the arch requires 64 bits to be 8 bytes > > > aligned. The alignment is meaningless here. All we need is "packed" and > > > be done with it. It's only going to truncate the 4 bytes at the end of > > > the structure if that. > > > > I agree that in-struct alignment preserved without the 'aligned' > > attribute but I'm not sure whether it's guaranteed that the *start* > > address of the struct is still in proper alignment boundary. > > > > IOW the struct ftrace_graph_ret should be placed at 8-byte boundary in order > > to keep alignment of struct members. Is it guaranteed after applying > > 'packed'? > > > > My point is, a structure doesn't change size depending on where it is > located. Thus, if a structure contains a 8 byte field, that must be 8 > bytes aligned due to architecture constraints, then it had better be > aligned that way everywhere. If it is not, then accessing the 8 byte > fields will cause issues. > > The only thing that the "packed" changes, is it removes the last 4 > padded bytes of the structure. It doesn't change anything else. Hence, > the alignment is just extra and unneeded. Ok then, I'll change it to use 'packed' only in v3. Thanks, Namhyung