From patchwork Thu Mar 30 19:27:32 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Marcelo Tosatti X-Patchwork-Id: 77381 Return-Path: Delivered-To: ouuuleilei@gmail.com Received: by 2002:a59:b0ea:0:b0:3b6:4342:cba0 with SMTP id b10csp93709vqo; Thu, 30 Mar 2023 12:46:20 -0700 (PDT) X-Google-Smtp-Source: AKy350YgJ4kfDaZy3CcweyFgB/rAauAwEbNZiyWhNZ1Tg6ri4un40lzu13pVDZedkDluiQES833o X-Received: by 2002:a17:907:1c21:b0:8b1:7ead:7d43 with SMTP id nc33-20020a1709071c2100b008b17ead7d43mr33229284ejc.50.1680205580073; Thu, 30 Mar 2023 12:46:20 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1680205580; cv=none; d=google.com; s=arc-20160816; b=tqJSqwJ1hoAK+5BY6q3yzZxSLAhBO97Wbc5T3v81aVJNOH6p9DJeWZ3U97PBcry2CY x5K2xUjyUjvmaGzfOQt0SAtI7N5D+E3waOqhGWS3yfqVa7ltHjhze7IF4FA1OUuGVGRr eEbOlnMaQAACgBbh8HXD/LZClUllI8+DqJasM1YY5KZrxylXqM2PG0hzJIkHo7MOu3IJ axmHD9I31jzXkrk/JzuKx2OudmTpRth1ZDbkb7dfTlJ+p2zCCxbmNh+1hs/MtGGbTzQH AIuPQxJpqNSYKbWSpdmaWx7Eo2+/veuxsZ0uirfjLfgAK6ERUtgQXjx5IyTTtZPmhw6U PqqQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:content-transfer-encoding:content-disposition :mime-version:message-id:subject:cc:to:from:date:dkim-signature; bh=/w6a9DZ97xRlu69CErtgPRkIVJNOsN242/AwObgPQ/M=; b=KMv88UxVe2J35PS+eVXMH+3AUZM+UR6gcCwLLH+fpkyvBaj+yVj9xFQGTMOrqA6r4u vBrcilIdBElTNOJFrV36RECNQPbvh1eWUwRmgvPUSnL4rFY4hVDlfQvbwDpk7TDVsIbR DYWFnezSJ2KjiyDuC19lLMPVUTNpp886K+OEqhlWJdjIsIVUguv1QGMYz6g+Xbj8zI0i llm4wUe8RGnLCbXeZzvC7mPNMLX+CuS4kDWAs0Pqaxieq5HRILwWFDXxROD7h/5tFxaC wi8/lwAp5BybkOmMy0z63FLDc7bIBrnpQf+2gTj3i42JdmZAndqA+zAsQlqr727rFLqD V8zg== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@redhat.com header.s=mimecast20190719 header.b=G9Ewg2s9; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from out1.vger.email (out1.vger.email. [2620:137:e000::1:20]) by mx.google.com with ESMTP id ca24-20020a170906a3d800b0094762fa0f07si276746ejb.720.2023.03.30.12.45.56; Thu, 30 Mar 2023 12:46:20 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; dkim=pass header.i=@redhat.com header.s=mimecast20190719 header.b=G9Ewg2s9; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231727AbjC3T3R (ORCPT + 99 others); Thu, 30 Mar 2023 15:29:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33512 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229902AbjC3T3Q (ORCPT ); Thu, 30 Mar 2023 15:29:16 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9B15910426 for ; Thu, 30 Mar 2023 12:28:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1680204506; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=/w6a9DZ97xRlu69CErtgPRkIVJNOsN242/AwObgPQ/M=; b=G9Ewg2s9ziGfeumAobkVrJ1fvG0+MTZ37CBOdtesYg7lZinV5UY7sIGZYlf2M3GI8ClMQW cnbCbNVpTAdTjHmwK1p8OMiMz05WbzcNt7mjrjFiIRPKxjw2U5EHh9DKDI6DD/2t7ZtD4N zwV72b0GDCka+8r7xtzz2bFaNsxJyuU= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-203-hH9XuhwSO-a9e57TJO9n4A-1; Thu, 30 Mar 2023 15:28:22 -0400 X-MC-Unique: hH9XuhwSO-a9e57TJO9n4A-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 27FF21C0758A; Thu, 30 Mar 2023 19:28:22 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9C50C4020C82; Thu, 30 Mar 2023 19:28:21 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 077814038BEC0; Thu, 30 Mar 2023 16:27:32 -0300 (-03) Date: Thu, 30 Mar 2023 16:27:32 -0300 From: Marcelo Tosatti To: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Cc: Alexander Viro , Matthew Wilcox , Christoph Hellwig , Jens Axboe , Frederic Weisbecker , Leonardo Bras , Yair Podemsky , P J P Subject: [PATCH v4] fs/buffer.c: update per-CPU bh_lru cache via RCU Message-ID: MIME-Version: 1.0 Content-Disposition: inline X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 X-Spam-Status: No, score=-0.2 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1761823246369404299?= X-GMAIL-MSGID: =?utf-8?q?1761823246369404299?= For certain types of applications (for example PLC software or RAN processing), upon occurrence of an event, it is necessary to complete a certain task in a maximum amount of time (deadline). One way to express this requirement is with a pair of numbers, deadline time and execution time, where: * deadline time: length of time between event and deadline. * execution time: length of time it takes for processing of event to occur on a particular hardware platform (uninterrupted). The particular values depend on use-case. For the case where the realtime application executes in a virtualized guest, an IPI which must be serviced in the host will cause the following sequence of events: 1) VM-exit 2) execution of IPI (and function call) 3) VM-entry Which causes an excess of 50us latency as observed by cyclictest (this violates the latency requirement of vRAN application with 1ms TTI, for example). invalidate_bh_lrus calls an IPI on each CPU that has non empty per-CPU cache: on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1); To avoid the IPI, free the per-CPU caches remotely via RCU. Two bh_lrus structures for each CPU are allocated: one is being used (assigned to per-CPU bh_lru pointer), and the other is being freed (or idle). An alternative solution would be to protect the fast path (__find_get_block) with a per-CPU spinlock. Then grab the lock from invalidate_bh_lru, when evaluating whether a given CPUs buffer_head cache should be invalidated. This solution would slow down the fast path. Numbers (16 vCPU guest) for the following test: for i in `seq 0 50`; mount -o loop alpine-standard-3.17.1-x86_64.iso /mnt/loop umount /mnt/loop done Where the time being measured is time between invalidate_bh_lrus function call start and return. Unpatched: average is 2us ┌ ┐ [ 0.0, 2.0) ┤████████████████████████▊ 53 [ 2.0, 4.0) ┤████████████████████████████████████ 77 [ 4.0, 6.0) ┤████████▍ 18 [ 6.0, 8.0) ┤▌ 1 [ 8.0, 10.0) ┤ 0 [10.0, 12.0) ┤ 0 [12.0, 14.0) ┤▌ 1 [14.0, 16.0) ┤ 0 [16.0, 18.0) ┤▌ 1 └ ┘ Frequency Patched: average is 16us ┌ ┐ [ 0.0, 10.0) ┤██████████████████▍ 35 [10.0, 20.0) ┤████████████████████████████████████ 69 [20.0, 30.0) ┤██████████████████▍ 35 [30.0, 40.0) ┤████▎ 8 [40.0, 50.0) ┤█▌ 3 [50.0, 60.0) ┤█▏ 2 └ ┘ Frequency The fact that invalidate_bh_lru() is now serialized should not be an issue, since invalidate_bdev does: /* Invalidate clean unused buffers and pagecache. */ void invalidate_bdev(struct block_device *bdev) { struct address_space *mapping = bdev->bd_inode->i_mapping; if (mapping->nrpages) { invalidate_bh_lrus(); lru_add_drain_all(); /* make sure all lru add caches are flushed */ invalidate_mapping_pages(mapping, 0, -1); } } Where lru_add_drain_all() is serialized by a single mutex lock (and there have been no reported use cases where this serialization is an issue). Regarding scalability, considering the results above where it takes 16us to execute invalidate_bh_lrus on 16 CPUs (where 8us are taken by synchronize_rcu_expedited), we can assume 500ns per CPU. For a system with 1024 CPUs, we can infer 8us + 1024*500ns ~= 500us (which seems acceptable). Signed-off-by: Marcelo Tosatti --- v4: improved changelog, no code change (Dave Chinner) v3: fix CPU hotplug v2: fix sparse warnings (kernel test robot) diff --git a/fs/buffer.c b/fs/buffer.c index 9e1e2add541e..e9b4d579eff0 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -1246,7 +1246,21 @@ struct bh_lru { struct buffer_head *bhs[BH_LRU_SIZE]; }; -static DEFINE_PER_CPU(struct bh_lru, bh_lrus) = {{ NULL }}; + +/* + * Allocate two bh_lrus structures for each CPU. bh_lru points to the + * one that is currently in use, and the update path does + * (consider cpu->bh_lru = bh_lrus[0]). + * + * cpu->bh_lrup = bh_lrus[1] + * synchronize_rcu() + * free bh's in bh_lrus[0] + */ +static unsigned int bh_lru_idx; +static DEFINE_PER_CPU(struct bh_lru, bh_lrus[2]) = {{{ NULL }}, {{NULL}}}; +static DEFINE_PER_CPU(struct bh_lru __rcu *, bh_lrup); + +static DEFINE_MUTEX(bh_lru_invalidate_mutex); #ifdef CONFIG_SMP #define bh_lru_lock() local_irq_disable() @@ -1288,16 +1302,19 @@ static void bh_lru_install(struct buffer_head *bh) return; } - b = this_cpu_ptr(&bh_lrus); + rcu_read_lock(); + b = rcu_dereference(per_cpu(bh_lrup, smp_processor_id())); for (i = 0; i < BH_LRU_SIZE; i++) { swap(evictee, b->bhs[i]); if (evictee == bh) { + rcu_read_unlock(); bh_lru_unlock(); return; } } get_bh(bh); + rcu_read_unlock(); bh_lru_unlock(); brelse(evictee); } @@ -1309,28 +1326,32 @@ static struct buffer_head * lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size) { struct buffer_head *ret = NULL; + struct bh_lru *lru; unsigned int i; check_irqs_on(); bh_lru_lock(); + rcu_read_lock(); + + lru = rcu_dereference(per_cpu(bh_lrup, smp_processor_id())); for (i = 0; i < BH_LRU_SIZE; i++) { - struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]); + struct buffer_head *bh = lru->bhs[i]; if (bh && bh->b_blocknr == block && bh->b_bdev == bdev && bh->b_size == size) { if (i) { while (i) { - __this_cpu_write(bh_lrus.bhs[i], - __this_cpu_read(bh_lrus.bhs[i - 1])); + lru->bhs[i] = lru->bhs[i - 1]; i--; } - __this_cpu_write(bh_lrus.bhs[0], bh); + lru->bhs[0] = bh; } get_bh(bh); ret = bh; break; } } + rcu_read_unlock(); bh_lru_unlock(); return ret; } @@ -1424,35 +1445,54 @@ static void __invalidate_bh_lrus(struct bh_lru *b) b->bhs[i] = NULL; } } -/* - * invalidate_bh_lrus() is called rarely - but not only at unmount. - * This doesn't race because it runs in each cpu either in irq - * or with preempt disabled. - */ -static void invalidate_bh_lru(void *arg) -{ - struct bh_lru *b = &get_cpu_var(bh_lrus); - - __invalidate_bh_lrus(b); - put_cpu_var(bh_lrus); -} bool has_bh_in_lru(int cpu, void *dummy) { - struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu); + struct bh_lru *b; int i; - + + rcu_read_lock(); + b = rcu_dereference(per_cpu(bh_lrup, cpu)); for (i = 0; i < BH_LRU_SIZE; i++) { - if (b->bhs[i]) + if (b->bhs[i]) { + rcu_read_unlock(); return true; + } } + rcu_read_unlock(); return false; } +/* + * invalidate_bh_lrus() is called rarely - but not only at unmount. + */ void invalidate_bh_lrus(void) { - on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1); + int cpu, oidx; + + mutex_lock(&bh_lru_invalidate_mutex); + cpus_read_lock(); + oidx = bh_lru_idx; + bh_lru_idx++; + if (bh_lru_idx >= 2) + bh_lru_idx = 0; + + /* Assign the per-CPU bh_lru pointer */ + for_each_online_cpu(cpu) + rcu_assign_pointer(per_cpu(bh_lrup, cpu), + per_cpu_ptr(&bh_lrus[bh_lru_idx], cpu)); + synchronize_rcu_expedited(); + + for_each_online_cpu(cpu) { + struct bh_lru *b = per_cpu_ptr(&bh_lrus[oidx], cpu); + + bh_lru_lock(); + __invalidate_bh_lrus(b); + bh_lru_unlock(); + } + cpus_read_unlock(); + mutex_unlock(&bh_lru_invalidate_mutex); } EXPORT_SYMBOL_GPL(invalidate_bh_lrus); @@ -1465,8 +1505,10 @@ void invalidate_bh_lrus_cpu(void) struct bh_lru *b; bh_lru_lock(); - b = this_cpu_ptr(&bh_lrus); + rcu_read_lock(); + b = rcu_dereference(per_cpu(bh_lrup, smp_processor_id())); __invalidate_bh_lrus(b); + rcu_read_unlock(); bh_lru_unlock(); } @@ -2968,15 +3010,25 @@ void free_buffer_head(struct buffer_head *bh) } EXPORT_SYMBOL(free_buffer_head); +static int buffer_cpu_online(unsigned int cpu) +{ + rcu_assign_pointer(per_cpu(bh_lrup, cpu), + per_cpu_ptr(&bh_lrus[bh_lru_idx], cpu)); + return 0; +} + static int buffer_exit_cpu_dead(unsigned int cpu) { int i; - struct bh_lru *b = &per_cpu(bh_lrus, cpu); + struct bh_lru *b; + rcu_read_lock(); + b = rcu_dereference(per_cpu(bh_lrup, cpu)); for (i = 0; i < BH_LRU_SIZE; i++) { brelse(b->bhs[i]); b->bhs[i] = NULL; } + rcu_read_unlock(); this_cpu_add(bh_accounting.nr, per_cpu(bh_accounting, cpu).nr); per_cpu(bh_accounting, cpu).nr = 0; return 0; @@ -3069,7 +3121,7 @@ EXPORT_SYMBOL(__bh_read_batch); void __init buffer_init(void) { unsigned long nrpages; - int ret; + int ret, cpu; bh_cachep = kmem_cache_create("buffer_head", sizeof(struct buffer_head), 0, @@ -3077,6 +3129,11 @@ void __init buffer_init(void) SLAB_MEM_SPREAD), NULL); + cpus_read_lock(); + for_each_online_cpu(cpu) + rcu_assign_pointer(per_cpu(bh_lrup, cpu), per_cpu_ptr(&bh_lrus[0], cpu)); + cpus_read_unlock(); + /* * Limit the bh occupancy to 10% of ZONE_NORMAL */ @@ -3085,4 +3142,7 @@ void __init buffer_init(void) ret = cpuhp_setup_state_nocalls(CPUHP_FS_BUFF_DEAD, "fs/buffer:dead", NULL, buffer_exit_cpu_dead); WARN_ON(ret < 0); + ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "fs/buffer:online", + NULL, buffer_cpu_online); + WARN_ON(ret < 0); }