Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq

[Waiman Long <longman@redhat.com>]

On 5/23/23 22:06, Yosry Ahmed wrote:
> Hi Ming,
>
> On Tue, May 23, 2023 at 6:21 PM Ming Lei <ming.lei@redhat.com> 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@redhat.com/
>>
>> Fixes: 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()")
>> Cc: Waiman Long <longman@redhat.com>
>> Cc: cgroups@vger.kernel.org
>> Cc: Tejun Heo <tj@kernel.org>
>> Cc: mkoutny@suse.com
>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>> ---
>>   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 = 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. 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 = per_cpu_ptr(blkcg->lhead, cpu);
>> +
>> +               if (!llist_empty(lhead))
>> +                       cgroup_rstat_css_cpu_flush(&blkcg->css, cpu);
>> +       }
>>
>>          /* release the blkcg and parent blkg refs this blkg has been holding */
>>          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 cpu);
>>
>>   /*
>>    * 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_rstat_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 
stat in the blkcg and it should be safe. I agree that we should note 
that in the comment to list the preconditions for calling it.

Cheers,
Longman