From patchwork Mon Nov 6 18:47:08 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Davidlohr Bueso X-Patchwork-Id: 10044251 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id CEE646032D for ; Mon, 6 Nov 2017 18:48:37 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id C0A3329F68 for ; Mon, 6 Nov 2017 18:48:37 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id B547D29F82; Mon, 6 Nov 2017 18:48:37 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 0E92229F68 for ; Mon, 6 Nov 2017 18:48:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932828AbdKFSsW (ORCPT ); Mon, 6 Nov 2017 13:48:22 -0500 Received: from mx2.suse.de ([195.135.220.15]:51430 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932327AbdKFSsT (ORCPT ); Mon, 6 Nov 2017 13:48:19 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 4E7DDAC9D; Mon, 6 Nov 2017 18:48:17 +0000 (UTC) Date: Mon, 6 Nov 2017 10:47:08 -0800 From: Davidlohr Bueso To: Waiman Long Cc: Alexander Viro , Jan Kara , Jeff Layton , "J. Bruce Fields" , Tejun Heo , Christoph Lameter , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra , Andi Kleen , Dave Chinner , Boqun Feng Subject: [PATCH v4] lib/dlock-list: Scale dlock_lists_empty() Message-ID: <20171106184708.kmwfcchjwjzucuja@linux-n805> References: <1509475860-16139-1-git-send-email-longman@redhat.com> <1509475860-16139-2-git-send-email-longman@redhat.com> <20171102170431.oq3i5mxtjcg53uot@linux-n805> <81bb3365-63f3-fea8-d238-e3880a4c8033@redhat.com> <20171103133420.pngmrsfmtimataz4@linux-n805> <20171103142254.d55bu2n44xe4aruf@linux-n805> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170421 (1.8.2) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Instead of the current O(N) implementation, at the cost of adding an atomic counter, we can convert the call to an atomic_read(). The counter only serves for accounting empty to non-empty transitions, and vice versa; therefore only modified twice for each of the lists during the lifetime of the dlock (while used). In addition, to be able to unaccount a list_del(), we add a dlist pointer to each head, thus minimizing the overall memory footprint. Signed-off-by: Davidlohr Bueso Acked-by: Waiman Long Reviewed-by: Jan Kara --- Changes from v3: - s/waiters/used_lists, more doc around the counter. - fixed racy scenario when the list empty/non-empty condition changes after taking the lock. - sprinkled unlikely() around all checks, these are only corner cases in the lifetime of the lock. include/linux/dlock-list.h | 8 ++++++ lib/dlock-list.c | 67 +++++++++++++++++++++++++++++++++++++++------- 2 files changed, 65 insertions(+), 10 deletions(-) diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h index c00c7f92ada4..e18690a9bba6 100644 --- a/include/linux/dlock-list.h +++ b/include/linux/dlock-list.h @@ -32,10 +32,18 @@ struct dlock_list_head { struct list_head list; spinlock_t lock; + struct dlock_list_heads *dlist; } ____cacheline_aligned_in_smp; +/* + * This is the main dlist data structure, with the array of heads + * and a counter that atomically tracks if any of the lists are + * being used. That is, empty to non-empty (and vice versa) + * head->list transitions. + */ struct dlock_list_heads { struct dlock_list_head *heads; + atomic_t used_lists; }; /* diff --git a/lib/dlock-list.c b/lib/dlock-list.c index a4ddecc01b12..a9c855d492b8 100644 --- a/lib/dlock-list.c +++ b/lib/dlock-list.c @@ -122,8 +122,11 @@ int __alloc_dlock_list_heads(struct dlock_list_heads *dlist, INIT_LIST_HEAD(&head->list); head->lock = __SPIN_LOCK_UNLOCKED(&head->lock); + head->dlist = dlist; lockdep_set_class(&head->lock, key); } + + atomic_set(&dlist->used_lists, 0); return 0; } EXPORT_SYMBOL(__alloc_dlock_list_heads); @@ -139,29 +142,36 @@ void free_dlock_list_heads(struct dlock_list_heads *dlist) { kfree(dlist->heads); dlist->heads = NULL; + atomic_set(&dlist->used_lists, 0); } EXPORT_SYMBOL(free_dlock_list_heads); /** * dlock_lists_empty - Check if all the dlock lists are empty * @dlist: Pointer to the dlock_list_heads structure - * Return: true if list is empty, false otherwise. * - * This can be a pretty expensive function call. If this function is required - * in a performance critical path, we may have to maintain a global count - * of the list entries in the global dlock_list_heads structure instead. + * Return: true if all dlock lists are empty, false otherwise. */ bool dlock_lists_empty(struct dlock_list_heads *dlist) { - int idx; - /* Shouldn't be called before nr_dlock_lists is initialized */ WARN_ON_ONCE(!nr_dlock_lists); - for (idx = 0; idx < nr_dlock_lists; idx++) - if (!list_empty(&dlist->heads[idx].list)) - return false; - return true; + /* + * Serialize dlist->used_lists such that a 0->1 transition is not + * missed by another thread checking if any of the dlock lists are + * used. + * + * CPU0 CPU1 + * dlock_list_add() dlock_lists_empty() + * [S] atomic_inc(used_lists); + * smp_mb__after_atomic(); + * smp_mb__before_atomic(); + * [L] atomic_read(used_lists) + * list_add() + */ + smp_mb__before_atomic(); + return !atomic_read(&dlist->used_lists); } EXPORT_SYMBOL(dlock_lists_empty); @@ -177,11 +187,39 @@ void dlock_lists_add(struct dlock_list_node *node, struct dlock_list_heads *dlist) { struct dlock_list_head *head = &dlist->heads[this_cpu_read(cpu2idx)]; + bool list_empty_before_lock = false; + + /* + * Optimistically bump the used_lists counter _before_ taking + * the head->lock such that we don't miss a thread adding itself + * to a list while spinning for the lock. + * + * Then, after taking the lock, recheck if the empty to non-empty + * transition changed and (un)account for ourselves, accordingly. + * Note that all these scenarios are corner cases, and not the + * common scenario, where the lists are actually populated most + * of the time. + */ + if (unlikely(list_empty_careful(&head->list))) { + list_empty_before_lock = true; + atomic_inc(&dlist->used_lists); + smp_mb__after_atomic(); + } /* * There is no need to disable preemption */ spin_lock(&head->lock); + + if (unlikely(!list_empty_before_lock && list_empty(&head->list))) { + atomic_inc(&dlist->used_lists); + smp_mb__after_atomic(); + } + if (unlikely(list_empty_before_lock && !list_empty(&head->list))) { + atomic_dec(&dlist->used_lists); + smp_mb__after_atomic(); + } + node->head = head; list_add(&node->list, &head->list); spin_unlock(&head->lock); @@ -212,6 +250,15 @@ void dlock_lists_del(struct dlock_list_node *node) spin_lock(&head->lock); if (likely(head == node->head)) { list_del_init(&node->list); + + if (unlikely(list_empty(&head->list))) { + struct dlock_list_heads *dlist; + dlist = node->head->dlist; + + atomic_dec(&dlist->used_lists); + smp_mb__after_atomic(); + } + node->head = NULL; retry = false; } else {