From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq Date: Wed, 24 May 2023 00:04:14 -0400 Message-ID: <11e81fc8-24db-54db-a518-b9bb67d0b504@redhat.com> References: <20230524011935.719659-1-ming.lei@redhat.com> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684901060; 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=RTp4n2rYhzZJCAzB2miuWTK/N8BXkz9FRgQIh2oQlBI=; b=Pj/EOwMGzWhmiQh/4P16yTlF8GK6IwlctRT9N5MunUutNEJ/y6A96mWqX994ICBNT1sIZu G+yo1BjXksuS7JyWvOQOFFgnp2FQZWKoEhsmCNs1on6tDIIfxRx24EyJvw6YClJxWwbDst VldJjszO76uY1oS1QHDdd+FNwgh+J9w= Content-Language: en-US In-Reply-To: List-ID: Content-Type: text/plain; charset="windows-1252"; format="flowed" To: Yosry Ahmed , Ming Lei , Linux-MM , Michal Hocko , Shakeel Butt , Johannes Weiner , Roman Gushchin , Muchun Song Cc: Jens Axboe , linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Tejun Heo , mkoutny-IBi9RG/b67k@public.gmane.org On 5/23/23 22:06, Yosry Ahmed wrote: > Hi Ming, > > On Tue, May 23, 2023 at 6:21=E2=80=AFPM 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. > I am not at all familiar with blkcg, but does calling > cgroup_rstat_flush() in offline_css() fix the problem? or can items be > added to the lockless list(s) after the blkcg is offlined? > >> 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: >> >> - a new cgroup_rstat_css_cpu_flush() function is added to flush stats for >> a given css and cpu. This new function will be called in __blkg_release(= ). >> >> - don't grab bio->bi_blkg when adding the stats into blkcg's per-cpu >> stat list, and this kind of handling is the most fragile part of >> original patch >> >> Based on Waiman's patch: >> >> https://lore.kernel.org/linux-block/20221215033132.230023-3-longman-H+wX= aHxf7aLQT0dZR+AlfA@public.gmane.org/ >> >> Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()") >> Cc: Waiman Long >> Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org >> Cc: Tejun Heo >> Cc: mkoutny-IBi9RG/b67k@public.gmane.org >> Signed-off-by: Ming Lei >> --- >> block/blk-cgroup.c | 15 +++++++++++++-- >> include/linux/cgroup.h | 1 + >> kernel/cgroup/rstat.c | 18 ++++++++++++++++++ >> 3 files changed, 32 insertions(+), 2 deletions(-) >> >> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c >> index 0ce64dd73cfe..5437b6af3955 100644 >> --- a/block/blk-cgroup.c >> +++ b/block/blk-cgroup.c >> @@ -163,10 +163,23 @@ static void blkg_free(struct blkcg_gq *blkg) >> static void __blkg_release(struct rcu_head *rcu) >> { >> struct blkcg_gq *blkg =3D container_of(rcu, struct blkcg_gq, rc= u_head); >> + struct blkcg *blkcg =3D 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. Meantime no new bio can refer to this blkg any more given >> + * the refcnt is killed. >> + */ >> + for_each_possible_cpu(cpu) { >> + struct llist_head *lhead =3D per_cpu_ptr(blkcg->lhead, c= pu); >> + >> + if (!llist_empty(lhead)) >> + cgroup_rstat_css_cpu_flush(&blkcg->css, cpu); >> + } >> >> /* release the blkcg and parent blkg refs this blkg has been ho= lding */ >> css_put(&blkg->blkcg->css); >> @@ -991,7 +1004,6 @@ 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: >> @@ -2075,7 +2087,6 @@ void blk_cgroup_bio_start(struct bio *bio) >> >> llist_add(&bis->lnode, lhead); >> WRITE_ONCE(bis->lqueued, true); >> - percpu_ref_get(&bis->blkg->refcnt); >> } >> >> u64_stats_update_end_irqrestore(&bis->sync, flags); >> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h >> index 885f5395fcd0..97d4764d8e6a 100644 >> --- a/include/linux/cgroup.h >> +++ b/include/linux/cgroup.h >> @@ -695,6 +695,7 @@ void cgroup_rstat_flush(struct cgroup *cgrp); >> void cgroup_rstat_flush_atomic(struct cgroup *cgrp); >> void cgroup_rstat_flush_hold(struct cgroup *cgrp); >> void cgroup_rstat_flush_release(void); >> +void cgroup_rstat_css_cpu_flush(struct cgroup_subsys_state *css, int cp= u); >> >> /* >> * Basic resource stats. >> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c >> index 9c4c55228567..96e7a4e6da72 100644 >> --- a/kernel/cgroup/rstat.c >> +++ b/kernel/cgroup/rstat.c >> @@ -281,6 +281,24 @@ void cgroup_rstat_flush_release(void) >> spin_unlock_irq(&cgroup_rstat_lock); >> } >> >> +/** >> + * cgroup_rstat_css_cpu_flush - flush stats for the given css and cpu >> + * @css: target css to be flush >> + * @cpu: the cpu that holds the stats to be flush >> + * >> + * A lightweight rstat flush operation for a given css and cpu. >> + * Only the cpu_lock is being held for mutual exclusion, the cgroup_rst= at_lock >> + * isn't used. > (Adding linux-mm and memcg maintainers) > +Linux-MM +Michal Hocko +Shakeel Butt +Johannes Weiner +Roman Gushchin > +Muchun Song > > I don't think flushing the stats without holding cgroup_rstat_lock is > safe for memcg stats flushing. mem_cgroup_css_rstat_flush() modifies > some non-percpu data (e.g. memcg->vmstats->state, > memcg->vmstats->state_pending). > > Perhaps have this be a separate callback than css_rstat_flush() (e.g. > css_rstat_flush_cpu() or something), so that it's clear what > subsystems support this? In this case, only blkcg would implement this > callback. That function is added to call blkcg_rstat_flush() only which flush the=20 stat in the blkcg and it should be safe. I agree that we should note=20 that in the comment to list the preconditions for calling it. Cheers, Longman