From patchwork Wed Jun 1 21:18:24 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Waiman Long X-Patchwork-Id: 12867315 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 856C5C433EF for ; Wed, 1 Jun 2022 21:18:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231336AbiFAVSr (ORCPT ); Wed, 1 Jun 2022 17:18:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51722 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231341AbiFAVSp (ORCPT ); Wed, 1 Jun 2022 17:18:45 -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 ESMTP id 3B79C70911 for ; Wed, 1 Jun 2022 14:18:42 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1654118321; 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=9crq163q6EkEQqxfbwAZtz3VF3ZbUXdixKOJuV9CM4M=; b=Gk90nfHlhKreX9oXggr1HK0wW+O9UH3GfK7ROkwTQ/MqCavOuEruHstQ6YX2wPrrJgld/N nFG9YvRN3zSWsFkrRcHV7z9GE3VN4hqzXy0c5rl+dGO5L5eTKu4r7Jq/MqphpSQ8charHN 14Wa7lkqQb2sgKmUSacjnFQZd4rq9No= 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-487-AeUu4TIwMG6v4JXg_p93HA-1; Wed, 01 Jun 2022 17:18:37 -0400 X-MC-Unique: AeUu4TIwMG6v4JXg_p93HA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7DFDD811E80; Wed, 1 Jun 2022 21:18:37 +0000 (UTC) Received: from llong.com (dhcp-17-215.bos.redhat.com [10.18.17.215]) by smtp.corp.redhat.com (Postfix) with ESMTP id 479F440CF8EB; Wed, 1 Jun 2022 21:18:37 +0000 (UTC) From: Waiman Long To: Tejun Heo , Jens Axboe Cc: cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Ming Lei , Waiman Long Subject: [PATCH v3 2/2] blk-cgroup: Optimize blkcg_rstat_flush() Date: Wed, 1 Jun 2022 17:18:24 -0400 Message-Id: <20220601211824.89626-3-longman@redhat.com> In-Reply-To: <20220601211824.89626-1-longman@redhat.com> References: <20220601211824.89626-1-longman@redhat.com> MIME-Version: 1.0 Content-type: text/plain X-Scanned-By: MIMEDefang 2.84 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org For a system with many CPUs and block devices, the time to do blkcg_rstat_flush() from cgroup_rstat_flush() can be rather long. It can be especially problematic as interrupt is disabled during the flush. It was reported that it might take seconds to complete in some extreme cases leading to hard lockup messages. As it is likely that not all the percpu blkg_iostat_set's has been updated since the last flush, those stale blkg_iostat_set's don't need to be flushed in this case. This patch optimizes blkcg_rstat_flush() by keeping a lockless list of recently updated blkg_iostat_set's in a newly added percpu blkcg->lhead pointer. The blkg_iostat_set is added to the lockless list on the update side in blk_cgroup_bio_start(). It is removed from the lockless list when flushed in blkcg_rstat_flush(). Due to racing, it is possible that blk_iostat_set's in the lockless list may have no new IO stats to be flushed. To protect against destruction of blkg, a percpu reference is taken when putting into the lockless list and released when removed. A blkg_iostat_set can determine if it is in a lockless list by checking the content of its lnode.next pointer which will be non-NULL when in a lockless list. This requires the presence of a special llist_last sentinel node to be put at the end of the lockless list. When booting up an instrumented test kernel with this patch on a 2-socket 96-thread system with cgroup v2, out of the 2051 calls to cgroup_rstat_flush() after bootup, 1788 of the calls were exited immediately because of empty lockless list. After an all-cpu kernel build, the ratio became 6295424/6340513. That was more than 99%. Signed-off-by: Waiman Long Reported-by: kernel test robot --- block/blk-cgroup.c | 85 ++++++++++++++++++++++++++++++++++++++++++---- block/blk-cgroup.h | 9 +++++ 2 files changed, 88 insertions(+), 6 deletions(-) diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index acd9b0aa8dc8..0143dda589bd 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -59,6 +59,57 @@ static struct workqueue_struct *blkcg_punt_bio_wq; #define BLKG_DESTROY_BATCH_SIZE 64 +/* + * lnode.next of the last entry in a lockless list is NULL. To make it + * always non-NULL for lnode's, a sentinel node has to be put at the + * end of the lockless list. So all the percpu lhead's are initialized + * to point to that sentinel node. + */ +static struct llist_node llist_last; + +static inline bool blkcg_llist_empty(struct llist_head *lhead) +{ + return lhead->first == &llist_last; +} + +static inline void init_blkcg_llists(struct blkcg *blkcg) +{ + int cpu; + + for_each_possible_cpu(cpu) + per_cpu_ptr(blkcg->lhead, cpu)->first = &llist_last; +} + +static inline struct llist_node * +fetch_delete_blkcg_llist(struct llist_head *lhead) +{ + return xchg(&lhead->first, &llist_last); +} + +static inline struct llist_node * +fetch_delete_lnode_next(struct llist_node *lnode) +{ + struct llist_node *next = READ_ONCE(lnode->next); + struct blkcg_gq *blkg = llist_entry(lnode, struct blkg_iostat_set, + lnode)->blkg; + + WRITE_ONCE(lnode->next, NULL); + percpu_ref_put(&blkg->refcnt); + return next; +} + +/* + * The retrieved blkg_iostat_set is immediately marked as not in the + * lockless list by clearing its node->next pointer. It could be put + * back into the list by a parallel update before the iostat's are + * finally flushed including probably the new update. + */ +#define blkcg_llist_for_each_entry_safe(pos, node, nxt) \ + for (; (node != &llist_last) && \ + (pos = llist_entry(node, struct blkg_iostat_set, lnode), \ + nxt = fetch_delete_lnode_next(node), true); \ + node = nxt) + /** * blkcg_css - find the current css * @@ -236,8 +287,10 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q, blkg->blkcg = blkcg; u64_stats_init(&blkg->iostat.sync); - for_each_possible_cpu(cpu) + for_each_possible_cpu(cpu) { u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync); + per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg; + } for (i = 0; i < BLKCG_MAX_POLS; i++) { struct blkcg_policy *pol = blkcg_policy[i]; @@ -852,17 +905,23 @@ static void blkg_iostat_sub(struct blkg_iostat *dst, struct blkg_iostat *src) static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) { struct blkcg *blkcg = css_to_blkcg(css); - struct blkcg_gq *blkg; + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); + struct llist_node *lnode, *lnext; + struct blkg_iostat_set *bisc; /* Root-level stats are sourced from system-wide IO stats */ if (!cgroup_parent(css->cgroup)) return; + if (blkcg_llist_empty(lhead)) + return; + rcu_read_lock(); - hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) { + lnode = fetch_delete_blkcg_llist(lhead); + blkcg_llist_for_each_entry_safe(bisc, lnode, lnext) { + struct blkcg_gq *blkg = bisc->blkg; struct blkcg_gq *parent = blkg->parent; - struct blkg_iostat_set *bisc = per_cpu_ptr(blkg->iostat_cpu, cpu); struct blkg_iostat cur, delta; unsigned long flags; unsigned int seq; @@ -1192,6 +1251,11 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css) } } + blkcg->lhead = alloc_percpu_gfp(struct llist_head, GFP_KERNEL); + if (!blkcg->lhead) + goto free_blkcg; + init_blkcg_llists(blkcg); + for (i = 0; i < BLKCG_MAX_POLS ; i++) { struct blkcg_policy *pol = blkcg_policy[i]; struct blkcg_policy_data *cpd; @@ -1233,7 +1297,8 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css) for (i--; i >= 0; i--) if (blkcg->cpd[i]) blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]); - + free_percpu(blkcg->lhead); +free_blkcg: if (blkcg != &blkcg_root) kfree(blkcg); unlock: @@ -1997,6 +2062,7 @@ static int blk_cgroup_io_type(struct bio *bio) void blk_cgroup_bio_start(struct bio *bio) { + struct blkcg *blkcg = bio->bi_blkg->blkcg; int rwd = blk_cgroup_io_type(bio), cpu; struct blkg_iostat_set *bis; unsigned long flags; @@ -2015,9 +2081,16 @@ void blk_cgroup_bio_start(struct bio *bio) } bis->cur.ios[rwd]++; + if (!READ_ONCE(bis->lnode.next)) { + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); + + llist_add(&bis->lnode, lhead); + percpu_ref_get(&bis->blkg->refcnt); + } + u64_stats_update_end_irqrestore(&bis->sync, flags); if (cgroup_subsys_on_dfl(io_cgrp_subsys)) - cgroup_rstat_updated(bio->bi_blkg->blkcg->css.cgroup, cpu); + cgroup_rstat_updated(blkcg->css.cgroup, cpu); put_cpu(); } diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h index d4de0a35e066..2c36362a332e 100644 --- a/block/blk-cgroup.h +++ b/block/blk-cgroup.h @@ -18,6 +18,7 @@ #include #include #include +#include struct blkcg_gq; struct blkg_policy_data; @@ -43,6 +44,8 @@ struct blkg_iostat { struct blkg_iostat_set { struct u64_stats_sync sync; + struct llist_node lnode; + struct blkcg_gq *blkg; struct blkg_iostat cur; struct blkg_iostat last; }; @@ -97,6 +100,12 @@ struct blkcg { struct blkcg_policy_data *cpd[BLKCG_MAX_POLS]; struct list_head all_blkcgs_node; + + /* + * List of updated percpu blkg_iostat_set's since the last flush. + */ + struct llist_head __percpu *lhead; + #ifdef CONFIG_BLK_CGROUP_FC_APPID char fc_app_id[FC_APPID_LEN]; #endif