From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755434AbeEANkX (ORCPT ); Tue, 1 May 2018 09:40:23 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:51964 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752009AbeEANkW (ORCPT ); Tue, 1 May 2018 09:40:22 -0400 Date: Tue, 1 May 2018 08:40:20 -0500 From: Josh Poimboeuf To: Nadav Amit Cc: Peter Zijlstra , Ingo Molnar , LKML , Thomas Gleixner , Linus Torvalds Subject: Re: Suboptimal inline heuristics due to non-code sections Message-ID: <20180501134020.fonel3x6plea5xdt@treble> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.6.0.1 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 01, 2018 at 06:50:14AM +0000, Nadav Amit wrote: > When gcc considers the size of a function for inlining decisions, it > apparently considers *all* sections. Since the kernel extensively uses > sections for things other than code (e.g., exception-table, bug-table), the > optimality of these decisions seem questionable to me. > > The objtool’s sections may be the most extreme case, as these sections are > discarded, while their size still appears to be considered by the inlining > heuristics. It may be beneficial not to consider (some) the other sections > as well, as they do not affect code-caching but only increase the kernel > size. > > To illustrate the issue, consider the function copy_overflow(): > > 0xffffffff819315e0 <+0>: push %rbp > 0xffffffff819315e1 <+1>: mov %rsi,%rdx > 0xffffffff819315e4 <+4>: mov %edi,%esi > 0xffffffff819315e6 <+6>: mov $0xffffffff820bc4b8,%rdi > 0xffffffff819315ed <+13>: mov %rsp,%rbp > 0xffffffff819315f0 <+16>: callq 0xffffffff81089b70 <__warn_printk> > 0xffffffff819315f5 <+21>: ud2 > 0xffffffff819315f7 <+23>: pop %rbp > 0xffffffff819315f8 <+24>: retq > > This function seems to me as a great candidate for inlining. Yet, in my 4.16 > build (using gcc 7.2), I get 38 non-inlined instances of this function in > vmlinux. Forcing CONFIG_STACK_VALIDATION to be disabled reduces the number > non-inlined instances to 35. Removing, in addition, the data which is saved > in the __bug_table makes all the instances of the function to be inlined. > > Obviously this certain function can be set as __always_inline, but the inline > heuristics seems to me as wrongfully biased. > > What do you think? > > Is there a way to make gcc to ignore sections for its inlining heuristics? Good find. Playing around with one of the affected files (crypto/af_alg.o), if I make the .discard.reachable section empty by removing the text reference from the annotate_reachable() macro, then copy_overflow() still isn't inlined. But if I remove the section completely by removing the pushsection/popsection, then copy_overflow() gets inlined. So GCC's inlining decisions are somehow influenced by the existence of some random empty section. This definitely seems like a GCC bug to me. -- Josh