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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 C0573C7618F for ; Wed, 17 Jul 2019 15:09:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 941C621743 for ; Wed, 17 Jul 2019 15:09:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="hgNH4C3m" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727317AbfGQPJs (ORCPT ); Wed, 17 Jul 2019 11:09:48 -0400 Received: from merlin.infradead.org ([205.233.59.134]:37630 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726260AbfGQPJr (ORCPT ); Wed, 17 Jul 2019 11:09:47 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help: List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=SVWkaeTZUofn6ExxaVIGuAizGoZUu1XC7cjSP278ehA=; b=hgNH4C3meP6H4QR8nVYYGc/a0B ChHUKb4btr0qAx6C8fV9B8xFTttU5BMmi0L6YuyZ64y1yIRegluNaML0nKc+5l/8CQuVKgI20C/OV x9sInHemezlesMlXmU075RKKJtz9YcsfHCrG6tFv0U2oLGdDXqIM2XsUQKIposBfNnkc+kQ5NFgUC PgPjZzUUQ5NquuE4WRBPEoarvydOg6GiwXe8TXmfScxRR26HP2a7l8ZcvTWI90urs1qmoa/ds2VrB 1FrwevaggTNefAgs2WpgVJcEJe9ceT7ZJNvvnX0ntVY+ZigWIRIZdPYGY4sds6P/X9/3tMIErtBLB 3VyyNAgA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1hnlYR-0002yN-5C; Wed, 17 Jul 2019 15:09:15 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id DA14B20197A67; Wed, 17 Jul 2019 17:09:13 +0200 (CEST) Date: Wed, 17 Jul 2019 17:09:13 +0200 From: Peter Zijlstra To: Alex Kogan Cc: linux@armlinux.org.uk, mingo@redhat.com, will.deacon@arm.com, arnd@arndb.de, longman@redhat.com, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, bp@alien8.de, hpa@zytor.com, x86@kernel.org, guohanjun@huawei.com, jglauber@marvell.com, steven.sistare@oracle.com, daniel.m.jordan@oracle.com, dave.dice@oracle.com, rahul.x.yadav@oracle.com Subject: Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Message-ID: <20190717150913.GY3419@hirez.programming.kicks-ass.net> References: <20190715192536.104548-1-alex.kogan@oracle.com> <20190715192536.104548-4-alex.kogan@oracle.com> <20190716155022.GR3419@hirez.programming.kicks-ass.net> <193BBB31-F376-451F-BDE1-D4807140EB51@oracle.com> <20190716184724.GH3402@hirez.programming.kicks-ass.net> <20190717083944.GR3463@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 17, 2019 at 10:27:30AM -0400, Alex Kogan wrote: > > On Jul 17, 2019, at 4:39 AM, Peter Zijlstra wrote: > > static void cna_splice_tail(struct cna_node *cn, struct cna_node *head, struct cna_node *tail) > > { > > struct cna_node *list; > > > > /* remove [head,tail] */ > > WRITE_ONCE(cn->mcs.next, tail->mcs.next); > > tail->mcs.next = NULL; > > > > /* stick [head,tail] on the secondary list tail */ > > if (cn->mcs.locked <= 1) { > > /* create secondary list */ > > head->tail = tail; > > cn->mcs.locked = head->encoded_tail; > > } else { > > /* add to tail */ > > list = (struct cna_node *)decode_tail(cn->mcs.locked); > > list->tail->next = head; > > list->tail = tail; > > } > > } > > > > static struct cna_node *cna_find_next(struct mcs_spinlock *node) > > { > > struct cna_node *cni, *cn = (struct cna_node *)node; > > struct cna_node *head, *tail = NULL; > > > > /* find any next lock from 'our' node */ > > for (head = cni = (struct cna_node *)READ_ONCE(cn->mcs.next); > > cni && cni->node != cn->node; > > tail = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next)) > > ; > > > > /* when found, splice any skipped locks onto the secondary list */ > > if (cni && tail) > > cna_splice_tail(cn, head, tail); > > > > return cni; > > } > > > > How's that? > > This is almost perfect!! :) > > The above should work, but I think we should have a specialized fast-path for > checking the immediate next node in the main queue. This would be the common > case, once we splice ‘other’ nodes onto the secondary queue. In the above we > would go through four branches before returning from cna_find_next(). In the > following we would have just one: Right, but can you measure a difference? ;-) Anyway, no real objection, just playing devils advocate here. > > static struct cna_node *cna_find_next(struct mcs_spinlock *node) > > { > > struct cna_node *cn = (struct cna_node *)node; > struct cna_node *cni = (struct cna_node *)READ_ONCE(cn->mcs.next); > > > > struct cna_node *head, *tail = NULL; > > > /* fast path */ > if (cni->node == cn->node) > return cni; > > > /* find any next lock from 'our' node */ > > for (head = cn->mcs.next; head = cni, you just did that load :-) > > cni && cni->node != cn->node; > > tail = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next)) > > ; > > > > /* when found, splice any skipped locks onto the secondary list */ > > if (cni && tail) > > cna_splice_tail(cn, head, tail); > > > > return cni; > > } > > > Also, any reason you say ‘lock’ instead of ’node’ in the comments? > I.e., I think "when found, splice any skipped locks onto the secondary list” should be > "when found, splice any skipped nodes onto the secondary list”. Due to the confusion between lock waiter node and numa node :-)