Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752832Ab3IJNv7 (ORCPT ); Tue, 10 Sep 2013 09:51:59 -0400 Received: from mail-bk0-f43.google.com ([209.85.214.43]:63051 "EHLO mail-bk0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751941Ab3IJNv5 (ORCPT ); Tue, 10 Sep 2013 09:51:57 -0400 Date: Tue, 10 Sep 2013 15:51:53 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Linus Torvalds , Andi Kleen , Peter Anvin , Mike Galbraith , Thomas Gleixner , Arjan van de Ven , Frederic Weisbecker , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: [PATCH 0/7] preempt_count rework -v2 Message-ID: <20130910135152.GD7537@gmail.com> References: <20130910130811.507933095@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130910130811.507933095@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4781 Lines: 107 * Peter Zijlstra wrote: > These patches optimize preempt_enable by firstly folding the preempt and > need_resched tests into one -- this should work for all architectures. And > secondly by providing per-arch preempt_count implementations; with x86 using > per-cpu preempt_count for fastest access. > > > These patches have been boot tested on CONFIG_PREEMPT=y x86_64 and survive > building a x86_64-defconfig kernel. > > kernel/sched/core.c:kick_process() now looks like: > > ffffffff8106f3f0 : > ffffffff8106f3f0: 55 push %rbp > ffffffff8106f3f1: 65 ff 04 25 e0 b7 00 incl %gs:0xb7e0 > ffffffff8106f3f8: 00 > ffffffff8106f3f9: 48 89 e5 mov %rsp,%rbp > ffffffff8106f3fc: 48 8b 47 08 mov 0x8(%rdi),%rax > ffffffff8106f400: 8b 50 18 mov 0x18(%rax),%edx > ffffffff8106f403: 65 8b 04 25 1c b0 00 mov %gs:0xb01c,%eax > ffffffff8106f40a: 00 > ffffffff8106f40b: 39 c2 cmp %eax,%edx > ffffffff8106f40d: 74 1b je ffffffff8106f42a > ffffffff8106f40f: 89 d1 mov %edx,%ecx > ffffffff8106f411: 48 c7 c0 00 2c 01 00 mov $0x12c00,%rax > ffffffff8106f418: 48 8b 0c cd a0 bc cb mov -0x7e344360(,%rcx,8),%rcx > ffffffff8106f41f: 81 > ffffffff8106f420: 48 3b bc 08 00 08 00 cmp 0x800(%rax,%rcx,1),%rdi > ffffffff8106f427: 00 > ffffffff8106f428: 74 1e je ffffffff8106f448 > * ffffffff8106f42a: 65 ff 0c 25 e0 b7 00 decl %gs:0xb7e0 > ffffffff8106f431: 00 > * ffffffff8106f432: 0f 94 c0 sete %al > * ffffffff8106f435: 84 c0 test %al,%al > * ffffffff8106f437: 75 02 jne ffffffff8106f43b > ffffffff8106f439: 5d pop %rbp > ffffffff8106f43a: c3 retq > * ffffffff8106f43b: e8 b0 b6 f9 ff callq ffffffff8100aaf0 <___preempt_schedule> Mind also posting the 'before' assembly, to make it clear how much we've improved things? > ffffffff8106f440: 5d pop %rbp > ffffffff8106f441: c3 retq > ffffffff8106f442: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) > ffffffff8106f448: 89 d7 mov %edx,%edi > ffffffff8106f44a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) > ffffffff8106f450: ff 15 ea e0 ba 00 callq *0xbae0ea(%rip) # ffffffff81c1d540 > ffffffff8106f456: eb d2 jmp ffffffff8106f42a > ffffffff8106f458: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) > ffffffff8106f45f: 00 > > Where the '*' marked lines are preempt_enable(), sadly GCC isn't able to > get rid of the sete+test :/ Its a rather frequent pattern in the kernel, > so 'fixing' the x86 GCC backend to recognise this might be useful. So what we do in kick_process() is: preempt_disable(); cpu = task_cpu(p); if ((cpu != smp_processor_id()) && task_curr(p)) smp_send_reschedule(cpu); preempt_enable(); The preempt_disable() looks sweet: > ffffffff8106f3f1: 65 ff 04 25 e0 b7 00 incl %gs:0xb7e0 > ffffffff8106f3f8: 00 and the '*' you marked is the preempt_enable() portion, which, with your new code, looks like this: #define preempt_check_resched() \ do { \ if (unlikely(!*preempt_count_ptr())) \ preempt_schedule(); \ } while (0) Which GCC translates to: > * ffffffff8106f42a: 65 ff 0c 25 e0 b7 00 decl %gs:0xb7e0 > ffffffff8106f431: 00 > * ffffffff8106f432: 0f 94 c0 sete %al > * ffffffff8106f435: 84 c0 test %al,%al > * ffffffff8106f437: 75 02 jne ffffffff8106f43b So, is the problem that GCC cannot pass a 'CPU flags' state out of asm(), only an explicit (pseudo-)value, right? Ideally we'd like to have something like: > * ffffffff8106f42a: 65 ff 0c 25 e0 b7 00 decl %gs:0xb7e0 > ffffffff8106f431: 00 > * ffffffff8106f437: 75 02 jne ffffffff8106f43b right? Thanks, Ingo -- 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/