From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752959AbcHNK1j (ORCPT ); Sun, 14 Aug 2016 06:27:39 -0400 Received: from mail-oi0-f49.google.com ([209.85.218.49]:35996 "EHLO mail-oi0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751229AbcHNK1i (ORCPT ); Sun, 14 Aug 2016 06:27:38 -0400 MIME-Version: 1.0 In-Reply-To: References: <1471106302-10159-1-git-send-email-brgerst@gmail.com> From: Brian Gerst Date: Sat, 13 Aug 2016 14:15:11 -0400 Message-ID: Subject: Re: [PATCH v3 0/7] x86: Rewrite switch_to() To: Linus Torvalds Cc: "the arch/x86 maintainers" , Linux Kernel Mailing List , Ingo Molnar , "H. Peter Anvin" , Denys Vlasenko , Andy Lutomirski , Borislav Petkov , Thomas Gleixner , Josh Poimboeuf Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 13, 2016 at 1:16 PM, Linus Torvalds wrote: > On Sat, Aug 13, 2016 at 9:38 AM, Brian Gerst wrote: >> This patch set simplifies the switch_to() code, by moving the stack switch >> code out of line into an asm stub before calling __switch_to(). This ends >> up being more readable, and using the C calling convention instead of >> clobbering all registers improves code generation. It also allows newly >> forked processes to construct a special stack frame to seamlessly flow >> to ret_from_fork, instead of using a test and branch, or an unbalanced >> call/ret. > > Do you have performance numbers? Is it noticeable/measurable? How do I measure it? The perf documentation isn't easy to understand. It shouldn't be a significant change. On a 64-bit defconfig build, __schedule() shrinks by 103 bytes. It's hard to analyse what exactly changes, but it's likely that GCC can allocate registers better without all the clobbers of the old inline asm version interfering. The new stub adds just 39 bytes. -- Brian Gerst