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 7DCDDC77B7C for ; Wed, 24 May 2023 15:46:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235414AbjEXPqo (ORCPT ); Wed, 24 May 2023 11:46:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44010 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235966AbjEXPqn (ORCPT ); Wed, 24 May 2023 11:46:43 -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 39B291A1 for ; Wed, 24 May 2023 08:45:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684943030; 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=go9RJx+lgX4qOrtsLx/9OhFpQNTZr1n8I6Ri6mX7g3A=; b=OY7Ji2etzLebMoU0QM/1Q6JsBoiZsez7PNU4e1MKFrtTjGQb3W1JvA48PbE/fSNSQcx3Ga 4PEWqMbtntNiDkBL5TOWCcMP6D2mD3sV4YkHhZiab99ak/Q0tEMjTxcmHFk2OAL8ZaM5cO fDkjO2EZt+NDoRgy2vWRp3i9a300oDo= 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-553-BDVEDZY2OeKAoesJqrJYuA-1; Wed, 24 May 2023 11:43:44 -0400 X-MC-Unique: BDVEDZY2OeKAoesJqrJYuA-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 56CC7185A792; Wed, 24 May 2023 15:43:44 +0000 (UTC) Received: from [10.22.17.224] (unknown [10.22.17.224]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0C6E240C6EC4; Wed, 24 May 2023 15:43:43 +0000 (UTC) Message-ID: <76a863b4-112e-82ae-59e4-6326fff48ffc@redhat.com> Date: Wed, 24 May 2023 11:43:43 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH V2] blk-cgroup: Flush stats before releasing blkcg_gq Content-Language: en-US To: Ming Lei Cc: Jens Axboe , linux-block@vger.kernel.org, Tejun Heo , mkoutny@suse.com, Yosry Ahmed References: <20230524035150.727407-1-ming.lei@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 5/24/23 00:26, Ming Lei wrote: > On Wed, May 24, 2023 at 12:19:57AM -0400, Waiman Long wrote: >> On 5/23/23 23:51, Ming Lei wrote: >>> As noted by Michal, the blkg_iostat_set's in the lockless list hold >>> reference to blkg's to protect against their removal. Those blkg's >>> hold reference to blkcg. When a cgroup is being destroyed, >>> cgroup_rstat_flush() is only called at css_release_work_fn() which >>> is called when the blkcg reference count reaches 0. This circular >>> dependency will prevent blkcg and some blkgs from being freed after >>> they are made offline. >>> >>> It is less a problem if the cgroup to be destroyed also has other >>> controllers like memory that will call cgroup_rstat_flush() which will >>> clean up the reference count. If block is the only controller that uses >>> rstat, these offline blkcg and blkgs may never be freed leaking more >>> and more memory over time. >>> >>> To prevent this potential memory leak: >>> >>> - flush blkcg per-cpu stats list in __blkg_release(), when no new stat >>> can be added >>> >>> - don't grab bio->bi_blkg reference when adding the stats into blkcg's >>> per-cpu stat list since all stats are guaranteed to be consumed before >>> releasing blkg instance, and grabbing blkg reference for stats was the >>> most fragile part of original patch >>> >>> Based on Waiman's patch: >>> >>> https://lore.kernel.org/linux-block/20221215033132.230023-3-longman@redhat.com/ >>> >>> Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()") >>> Cc: Waiman Long >>> Cc: Tejun Heo >>> Cc: mkoutny@suse.com >>> Cc: Yosry Ahmed >>> Signed-off-by: Ming Lei >>> --- >>> V2: >>> - remove kernel/cgroup change, and call blkcg_rstat_flush() >>> to flush stat directly >>> >>> block/blk-cgroup.c | 29 +++++++++++++++++++++-------- >>> 1 file changed, 21 insertions(+), 8 deletions(-) >>> >>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c >>> index 0ce64dd73cfe..ed0eb8896972 100644 >>> --- a/block/blk-cgroup.c >>> +++ b/block/blk-cgroup.c >>> @@ -34,6 +34,8 @@ >>> #include "blk-ioprio.h" >>> #include "blk-throttle.h" >>> +static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu); >>> + >>> /* >>> * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation. >>> * blkcg_pol_register_mutex nests outside of it and synchronizes entire >>> @@ -163,10 +165,21 @@ static void blkg_free(struct blkcg_gq *blkg) >>> static void __blkg_release(struct rcu_head *rcu) >>> { >>> struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head); >>> + struct blkcg *blkcg = blkg->blkcg; >>> + int cpu; >>> #ifdef CONFIG_BLK_CGROUP_PUNT_BIO >>> WARN_ON(!bio_list_empty(&blkg->async_bios)); >>> #endif >>> + /* >>> + * Flush all the non-empty percpu lockless lists before releasing >>> + * us, given these stat belongs to us. >>> + * >>> + * cgroup locks aren't needed here since __blkcg_rstat_flush just >>> + * propagates delta into blkg parent, which is live now. >>> + */ >>> + for_each_possible_cpu(cpu) >>> + __blkcg_rstat_flush(blkcg, cpu); >>> /* release the blkcg and parent blkg refs this blkg has been holding */ >>> css_put(&blkg->blkcg->css); >>> @@ -951,17 +964,12 @@ static void blkcg_iostat_update(struct blkcg_gq *blkg, struct blkg_iostat *cur, >>> u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags); >>> } >>> -static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) >>> +static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu) >>> { >>> - struct blkcg *blkcg = css_to_blkcg(css); >>> struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu); >>> struct llist_node *lnode; >>> struct blkg_iostat_set *bisc, *next_bisc; >>> - /* Root-level stats are sourced from system-wide IO stats */ >>> - if (!cgroup_parent(css->cgroup)) >>> - return; >>> - >>> rcu_read_lock(); >>> lnode = llist_del_all(lhead); >>> @@ -991,13 +999,19 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) >>> if (parent && parent->parent) >>> blkcg_iostat_update(parent, &blkg->iostat.cur, >>> &blkg->iostat.last); >>> - percpu_ref_put(&blkg->refcnt); >>> } >>> out: >>> rcu_read_unlock(); >>> } >>> +static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) >>> +{ >>> + /* Root-level stats are sourced from system-wide IO stats */ >>> + if (cgroup_parent(css->cgroup)) >>> + __blkcg_rstat_flush(css_to_blkcg(css), cpu); >>> +} >>> + >> I think it may not safe to call __blkcg_rstat_flus() directly without taking >> the cgroup_rstat_cpu_lock. That is why I added a helper to >> kernel/cgroup/rstat.c in my patch to meet the locking requirement. > All stats are removed from llist_del_all(), and the local list is > iterated, then each blkg & its parent is touched in __blkcg_rstat_flus(), so > can you explain it a bit why cgroup locks are needed? For protecting > what? You are right. The llist_del_all() call in blkcg_rstat_flush() is atomic, so it is safe for concurrent execution which is what the cgroup_rstat_cpu_lock protects against. That may not be the case for rstat callbacks of other controllers. So I will suggest you to add a comment to clarify that point. Other than that, you patch looks good to me. Reviewed: Waiman Long