From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759309AbZEKVF1 (ORCPT ); Mon, 11 May 2009 17:05:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756766AbZEKVFL (ORCPT ); Mon, 11 May 2009 17:05:11 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:36805 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756571AbZEKVFK (ORCPT ); Mon, 11 May 2009 17:05:10 -0400 Date: Mon, 11 May 2009 23:04:33 +0200 From: Ingo Molnar To: Vitaly Mayatskikh Cc: Christoph Hellwig , Hiroshi Shimamoto , Andrew Morton , Oleg Nesterov , Roland McGrath , linux-kernel@vger.kernel.org, "H. Peter Anvin" , Thomas Gleixner Subject: Re: [PATCH 1/5] Split wait_noreap_copyout() Message-ID: <20090511210433.GA9588@elte.hu> References: <1242036759-4025-1-git-send-email-v.mayatskih@gmail.com> <1242036759-4025-2-git-send-email-v.mayatskih@gmail.com> <20090511120418.GA3859@infradead.org> <20090511121708.GD13954@elte.hu> <87zldjwemo.wl%vmayatsk@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87zldjwemo.wl%vmayatsk@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Vitaly Mayatskikh wrote: > At Mon, 11 May 2009 14:17:08 +0200, Ingo Molnar wrote: > > > > if (put_user(signal, &infop->si_signo) || > > > put_user(0, &infop->si_errno) || > > > put_user((short)why, &infop->si_code) || > > > put_user(pid, &infop->si_pid) || > > > put_user(uid, &infop->si_uid) || > > > put_user(status, &infop->si_status)) > > > return -EFAULT; > > > > For best assembly code this should generally be written as a series > > of: > > > > __uaccess_err |= __put_user(x, ptr); > > __uaccess_err |= __put_user(y, ptr); > > __uaccess_err |= __put_user(z, ptr); > > > > As this generates non-dependent, compressed, branch-less code. > > Yeah, my first intention was to eliminate a lot of branches in one > place. It's terrible for CPU pipeline, I bet. > > > See the (new) put_user_try / put_user_ex() / put_user_catch() > > abstraction in arch/x86/include/asm/uaccess.h, and how all the > > x86 signal code makes use of that to optimize such patterns of > > per field user copies. > > So, there's catch block to handle GPF and the code inside of `try' > block is still branch-less, right? I was thinking of minimized > version of struct siginfo (up to si_uid) and copying it with > single copy_to_user(), but the idea with try/catch is definitely > much better. It creates really nice assembly code. Hiroshi-san experimented with it a lot until he found this form. Regarding potentially generalizing that facility into generic code, it relies on the exception code filling in current_thread_info()->uaccess_err with -EFAULT. So it needs architecture level support. It also kind of relies on current_thread_info()->uaccess_err being super-optimal - which it is on x86. (the assembler can optimize it) But a compatible wrapper could be added, for architectures that dont support, or that dont need support. Ingo