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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 627AAC77B75 for ; Wed, 3 May 2023 13:53:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229995AbjECNxz (ORCPT ); Wed, 3 May 2023 09:53:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58696 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229803AbjECNxy (ORCPT ); Wed, 3 May 2023 09:53:54 -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 99CA459F7 for ; Wed, 3 May 2023 06:53:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1683121985; 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: in-reply-to:in-reply-to:references:references; bh=IXVN9njL58i0gdsYraIjStXx8q1dlH84VTerQaHWg8E=; b=SUc55hwUdaHwyMfFUF9FMEBIrie/MHL+7CY8GnefM+OGO7r6GXd7uRVjgRmyT0fK5t+Mpj kAMnyrBKsGsQaSEikyUacnYX+cFljycSbQaODTFNalC/FNRjG2SzkjEZIX9bHqhqn8EOn+ 9UekJQc+bzUH4EDvYpxediMsmdPklmc= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-569-b2ch48CAOYKYY3aTTnc_8Q-1; Wed, 03 May 2023 09:53:00 -0400 X-MC-Unique: b2ch48CAOYKYY3aTTnc_8Q-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 2F60E103B7EA; Wed, 3 May 2023 13:53:00 +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 8049F1410DD7; Wed, 3 May 2023 13:52:59 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 8011C426F3D60; Wed, 3 May 2023 10:52:36 -0300 (-03) Date: Wed, 3 May 2023 10:52:36 -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: Re: [PATCH v4] fs/buffer.c: update per-CPU bh_lru cache via RCU Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Friendly ping ? On Thu, Mar 30, 2023 at 04:27:32PM -0300, Marcelo Tosatti wrote: > > 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); > } > >