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 D2FFCC77B73 for ; Wed, 24 May 2023 17:29:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232915AbjEXR3e (ORCPT ); Wed, 24 May 2023 13:29:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47792 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232808AbjEXR3d (ORCPT ); Wed, 24 May 2023 13:29:33 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6D565BB for ; Wed, 24 May 2023 10:28:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684949325; 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=MGTs+Zku8HtvEkwSQnMZU+mx6+G+RMJNFOMNVSUjxw4=; b=gk1cqtqtOsQQfzocOu61qIE7Rx1cl/vaz24LZ2xeePj0EnhDdKSpFxfeqsiLUGQK5cm3/q njpljioV8GQNm2159WHcecYPjogXgHQB8ChHaf3MWhLIcObNnvEPmhnJA0cvy0hq7QobhB y5t84ZbtGCAJ/JDiXGyfpjNpK49vms4= 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-126-icPyCQWwMXmw6ZSYxrAWUA-1; Wed, 24 May 2023 13:28:42 -0400 X-MC-Unique: icPyCQWwMXmw6ZSYxrAWUA-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 C626D3C00089; Wed, 24 May 2023 17:28:41 +0000 (UTC) Received: from [10.22.17.224] (unknown [10.22.17.224]) by smtp.corp.redhat.com (Postfix) with ESMTP id 730F540CFD45; Wed, 24 May 2023 17:28:41 +0000 (UTC) Message-ID: Date: Wed, 24 May 2023 13:28:41 -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 From: Waiman Long 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> <76a863b4-112e-82ae-59e4-6326fff48ffc@redhat.com> In-Reply-To: <76a863b4-112e-82ae-59e4-6326fff48ffc@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 5/24/23 11:43, Waiman Long wrote: > 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 After some more thought, I need to retract my reviewed-by tag for now. There is a slight possibility that blkcg_iostat_update() in blkcg_rstat_flush() can happen concurrently which will corrupt the sequence count. One way to avoid that is to synchronize it by cgroup_rstat_cpu_lock. Another way is to use the bisc->lqueued for synchronization. In that case, you need to move WRITE_ONCE(bisc->lqueued, false) in blkcg_rstat_flush() to the end after all the  blkcg_iostat_update() call with smp_store_release() and replace the READ_ONCE(bis->lqueued) check in blk_cgroup_bio_start() with smp_load_acquire(). Cheers, Longman