From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 85A7DC43331 for ; Tue, 12 Nov 2019 09:55:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 64222206BA for ; Tue, 12 Nov 2019 09:55:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727398AbfKLJz3 (ORCPT ); Tue, 12 Nov 2019 04:55:29 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:33033 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726008AbfKLJz2 (ORCPT ); Tue, 12 Nov 2019 04:55:28 -0500 Received: from p5b06da22.dip0.t-ipconnect.de ([91.6.218.34] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1iUStR-0007OB-Pn; Tue, 12 Nov 2019 10:55:25 +0100 Date: Tue, 12 Nov 2019 10:55:24 +0100 (CET) From: Thomas Gleixner To: Peter Zijlstra cc: LKML , x86@kernel.org, Linus Torvalds , Andy Lutomirski , Stephen Hemminger , Willy Tarreau , Juergen Gross , Sean Christopherson , "H. Peter Anvin" , Linus Torvalds Subject: Re: [patch V2 08/16] x86/ioperm: Add bitmap sequence numberc In-Reply-To: <20191112092246.GY4131@hirez.programming.kicks-ass.net> Message-ID: References: <20191111220314.519933535@linutronix.de> <20191111223052.292300453@linutronix.de> <20191112092246.GY4131@hirez.programming.kicks-ass.net> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 12 Nov 2019, Peter Zijlstra wrote: > On Mon, Nov 11, 2019 at 11:03:22PM +0100, Thomas Gleixner wrote: > > Add a globally unique sequence number which is incremented when ioperm() is > > changing the I/O bitmap of a task. Store the new sequence number in the > > io_bitmap structure and compare it along with the actual struct pointer > > with the one which was last loaded on a CPU. Only update the bitmap if > > either of the two changes. That should further reduce the overhead of I/O > > bitmap scheduling when there are only a few I/O bitmap users on the system. > > > + /* Update the sequence number to force an update in switch_to() */ > > + iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence); > > > + if (tss->last_bitmap != iobm || > > + tss->last_sequence != iobm->sequence) > > + switch_to_update_io_bitmap(tss, iobm); > > Initially I wondered why we need a globally unique sequence number if we > already check the struct iobitmap pointer. That ought to make the > sequence number per-object. > > However, that breaks for memory re-use. So yes, we need that thing to be > global. Actually with a global 64bit wide counter we can just avoid the pointer comparison. Assumed a ioperm() syscall every nanosecond it takes about 584 years of uptime to wrap around. :) Thanks, tglx