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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,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 45729C7618F for ; Tue, 16 Jul 2019 15:50:32 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1842D20693 for ; Tue, 16 Jul 2019 15:50:32 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="bHbTdeZ7" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1842D20693 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=3vqWdi6eg3gZLCnH2kCHdBsRkEgbViIik9UVbOkos0Y=; b=bHbTdeZ7yNGXTo UwmuMMEd+z8LHBqrt6A3OMWCB8PZwx32V6EddzVJnHl4x6oZsYUYxGLv22NWvmvlPywyJSF8Hjeh+ jz6w2wJrGnxvV7HKgktX8MhFYIe+0UHkbsizMAxBiGHWc0mNjIbpDGOT6uYpvV2efdugYLxHJFde+ BfE41aMlpY3psMmMead4B6B6VRV4swhdkg+2J7gfMz879xUe42dISwnVnOlD28BnTHJQngx4d5I2P h6KsoDGRGI1B7VRhiQLFw0tF90hhXn2jCdqHZz6VQzLZmR1xBH8yRB/aAzZzQaPCR9+VI9FnFXFwM WyyBOgfjaFbv1eRx6DuA==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hnPin-0005Wb-7G; Tue, 16 Jul 2019 15:50:29 +0000 Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1hnPii-0005WM-VJ; Tue, 16 Jul 2019 15:50:25 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id B879C20B15D63; Tue, 16 Jul 2019 17:50:22 +0200 (CEST) Date: Tue, 16 Jul 2019 17:50:22 +0200 From: Peter Zijlstra To: Alex Kogan Subject: Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Message-ID: <20190716155022.GR3419@hirez.programming.kicks-ass.net> References: <20190715192536.104548-1-alex.kogan@oracle.com> <20190715192536.104548-4-alex.kogan@oracle.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190715192536.104548-4-alex.kogan@oracle.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, guohanjun@huawei.com, arnd@arndb.de, dave.dice@oracle.com, jglauber@marvell.com, x86@kernel.org, will.deacon@arm.com, linux@armlinux.org.uk, steven.sistare@oracle.com, linux-kernel@vger.kernel.org, rahul.x.yadav@oracle.com, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, longman@redhat.com, tglx@linutronix.de, daniel.m.jordan@oracle.com, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, Jul 15, 2019 at 03:25:34PM -0400, Alex Kogan wrote: > +static struct cna_node *find_successor(struct mcs_spinlock *me) > +{ > + struct cna_node *me_cna = CNA_NODE(me); > + struct cna_node *head_other, *tail_other, *cur; > + struct cna_node *next = CNA_NODE(READ_ONCE(me->next)); > + int my_node; > + > + /* @next should be set, else we would not be calling this function. */ > + WARN_ON_ONCE(next == NULL); > + > + my_node = me_cna->numa_node; > + > + /* > + * Fast path - check whether the immediate successor runs on > + * the same node. > + */ > + if (next->numa_node == my_node) > + return next; > + > + head_other = next; > + tail_other = next; > + > + /* > + * Traverse the main waiting queue starting from the successor of my > + * successor, and look for a thread running on the same node. > + */ > + cur = CNA_NODE(READ_ONCE(next->mcs.next)); > + while (cur) { > + if (cur->numa_node == my_node) { > + /* > + * Found a thread on the same node. Move threads > + * between me and that node into the secondary queue. > + */ > + if (me->locked > 1) > + CNA_NODE(me->locked)->tail->mcs.next = > + (struct mcs_spinlock *)head_other; > + else > + me->locked = (uintptr_t)head_other; > + tail_other->mcs.next = NULL; > + CNA_NODE(me->locked)->tail = tail_other; > + return cur; > + } > + tail_other = cur; > + cur = CNA_NODE(READ_ONCE(cur->mcs.next)); > + } > + return NULL; > +} static void cna_move(struct cna_node *cn, struct cna_node *cni) { struct cna_node *head, *tail; /* remove @cni */ WRITE_ONCE(cn->mcs.next, cni->mcs.next); /* stick @cni on the 'other' list tail */ cni->mcs.next = NULL; if (cn->mcs.locked <= 1) { /* head = tail = cni */ head = cni; head->tail = cni; cn->mcs.locked = head->encoded_tail; } else { /* add to tail */ head = (struct cna_node *)decode_tail(cn->mcs.locked); tail = tail->tail; tail->next = cni; } } static struct cna_node *cna_find_next(struct mcs_spinlock *node) { struct cna_node *cni, *cn = (struct cna_node *)node; while ((cni = (struct cna_node *)READ_ONCE(cn->mcs.next))) { if (likely(cni->node == cn->node)) break; cna_move(cn, cni); } return cni; } > +static inline bool cna_set_locked_empty_mcs(struct qspinlock *lock, u32 val, > + struct mcs_spinlock *node) > +{ > + /* Check whether the secondary queue is empty. */ > + if (node->locked <= 1) { > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, > + _Q_LOCKED_VAL)) > + return true; /* No contention */ > + } else { > + /* > + * Pass the lock to the first thread in the secondary > + * queue, but first try to update the queue's tail to > + * point to the last node in the secondary queue. That comment doesn't make sense; there's at least one conditional missing. > + */ > + struct cna_node *succ = CNA_NODE(node->locked); > + u32 new = succ->tail->encoded_tail + _Q_LOCKED_VAL; > + > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) { > + arch_mcs_spin_unlock_contended(&succ->mcs.locked, 1); > + return true; > + } > + } > + > + return false; > +} static cna_try_clear_tail(struct qspinlock *lock, u32 val, struct mcs_spinlock *node) { if (node->locked <= 1) return __try_clear_tail(lock, val, node); /* the other case */ } > +static inline void cna_pass_mcs_lock(struct mcs_spinlock *node, > + struct mcs_spinlock *next) > +{ > + struct cna_node *succ = NULL; > + u64 *var = &next->locked; > + u64 val = 1; > + > + succ = find_successor(node); This makes unlock O(n), which is 'funneh' and undocumented. > + > + if (succ) { > + var = &succ->mcs.locked; > + /* > + * We unlock a successor by passing a non-zero value, > + * so set @val to 1 iff @locked is 0, which will happen > + * if we acquired the MCS lock when its queue was empty > + */ > + val = node->locked + (node->locked == 0); > + } else if (node->locked > 1) { /* if the secondary queue is not empty */ > + /* pass the lock to the first node in that queue */ > + succ = CNA_NODE(node->locked); > + succ->tail->mcs.next = next; > + var = &succ->mcs.locked; > + } /* > + * Otherwise, pass the lock to the immediate successor > + * in the main queue. > + */ I don't think this mis-indented comment can happen. The call-site guarantees @next is non-null. Therefore, cna_find_next() will either return it, or place it on the secondary list. If it (cna_find_next) returns NULL, we must have a non-empty secondary list. In no case do I see this tertiary condition being possible. > + > + arch_mcs_spin_unlock_contended(var, val); > +} This also renders this @next argument superfluous. static cna_mcs_pass_lock(struct mcs_spinlock *node, struct mcs_spinlock *next) { next = cna_find_next(node); if (!next) { BUG_ON(node->locked <= 1); next = (struct cna_node *)decode_tail(node->locked); node->locked = 1; } arch_mcs_pass_lock(&next->mcs.locked, node->locked); } _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel