Re: [PATCH v2] memcg: execute partial memcg freeing in mem_cgroup_destroy

[Glauber Costa <glommer@parallels.com>]

On 08/17/2012 08:51 AM, Greg Thelen wrote:
> On Thu, Aug 16 2012, Glauber Costa wrote:
> 
>> A lot of the initialization we do in mem_cgroup_create() is done with
>> softirqs enabled. This include grabbing a css id, which holds
>> &ss->id_lock->rlock, and the per-zone trees, which holds
>> rtpz->lock->rlock. All of those signal to the lockdep mechanism that
>> those locks can be used in SOFTIRQ-ON-W context. This means that the
>> freeing of memcg structure must happen in a compatible context,
>> otherwise we'll get a deadlock.
>>
>> The reference counting mechanism we use allows the memcg structure to be
>> freed later and outlive the actual memcg destruction from the
>> filesystem. However, we have little, if any, means to guarantee in which
>> context the last memcg_put will happen. The best we can do is test it
>> and try to make sure no invalid context releases are happening. But as
>> we add more code to memcg, the possible interactions grow in number and
>> expose more ways to get context conflicts.
>>
>> Greg Thelen reported a bug with that patchset applied that would trigger
>> if a task would hold a reference to a memcg through its kmem counter.
>> This would mean that killing that task would eventually get us to
>> __mem_cgroup_free() after dropping the last kernel page reference, in an
>> invalid IN-SOFTIRQ-W.
>>
>> Besides that, he raised the quite valid concern that keeping the full
>> memcg around for an unbounded period of time can eventually exhaust the
>> css_id space, and pin a lot of not needed memory. For instance, a
>> O(nr_cpus) percpu data for the stats is kept around, and we don't expect
>> to use it after the memcg is gone.
>>
>> Both those problems can be avoided by freeing as much as we can in
>> mem_cgroup_destroy(), and leaving only the memcg structure and the
>> static branches to be removed later. That freeing run on a predictable
>> context, getting rid of the softirq problem, and also reduces pressure
>> both on the css_id space and total dangling memory.
> 
> Thank you for the patch.  I think it is a step in the right direction,
> but I suspect a problem (noted below).
> 
>> I consider this safe because all the page_cgroup references to user
>> pages are reparented to the imediate parent, so late uncharges won't
>> trigger the common uncharge paths with a destroyed memcg.
>>
>> Although we don't migrate kernel pages to parent, we also don't call the
>> common uncharge paths for those pages, rather uncharging the
>> res_counters directly. So we are safe on this side of the wall as well.
>>
>> Signed-off-by: Glauber Costa <glommer@parallels.com>
>> Reported-by: Greg Thelen <gthelen@google.com>
>> CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> CC: Michal Hocko <mhocko@suse.cz>
>> CC: Johannes Weiner <hannes@cmpxchg.org>
>> ---
>>  mm/memcontrol.c | 25 ++++++++++++-------------
>>  1 file changed, 12 insertions(+), 13 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 9a82965..78cb394 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5169,18 +5169,9 @@ static void free_work(struct work_struct *work)
>>  		vfree(memcg);
>>  }
>>  
>> -static void free_rcu(struct rcu_head *rcu_head)
>> -{
>> -	struct mem_cgroup *memcg;
>> -
>> -	memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing);
>> -	INIT_WORK(&memcg->work_freeing, free_work);
>> -	schedule_work(&memcg->work_freeing);
>> -}
>> -
>>  /*
>> - * At destroying mem_cgroup, references from swap_cgroup can remain.
>> - * (scanning all at force_empty is too costly...)
>> + * At destroying mem_cgroup, references from swap_cgroup and other places can
>> + * remain.  (scanning all at force_empty is too costly...)
>>   *
>>   * Instead of clearing all references at force_empty, we remember
>>   * the number of reference from swap_cgroup and free mem_cgroup when
>> @@ -5188,6 +5179,14 @@ static void free_rcu(struct rcu_head *rcu_head)
>>   *
>>   * Removal of cgroup itself succeeds regardless of refs from swap.
>>   */
>> +static void free_rcu(struct rcu_head *rcu_head)
>> +{
>> +	struct mem_cgroup *memcg;
>> +
>> +	memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing);
>> +	INIT_WORK(&memcg->work_freeing, free_work);
>> +	schedule_work(&memcg->work_freeing);
>> +}
>>  
>>  static void __mem_cgroup_free(struct mem_cgroup *memcg)
>>  {
>> @@ -5200,7 +5199,6 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
>>  		free_mem_cgroup_per_zone_info(memcg, node);
>>  
>>  	free_percpu(memcg->stat);
>> -	call_rcu(&memcg->rcu_freeing, free_rcu);
>>  }
>>  
>>  static void mem_cgroup_get(struct mem_cgroup *memcg)
>> @@ -5212,7 +5210,7 @@ static void __mem_cgroup_put(struct mem_cgroup *memcg, int count)
>>  {
>>  	if (atomic_sub_and_test(count, &memcg->refcnt)) {
>>  		struct mem_cgroup *parent = parent_mem_cgroup(memcg);
>> -		__mem_cgroup_free(memcg);
>> +		call_rcu(&memcg->rcu_freeing, free_rcu);
>>  		if (parent)
>>  			mem_cgroup_put(parent);
>>  	}
>> @@ -5377,6 +5375,7 @@ static void mem_cgroup_destroy(struct cgroup *cont)
>>  
>>  	kmem_cgroup_destroy(memcg);
>>  
>> +	__mem_cgroup_free(memcg);
> 
> I suspect that this will free the css_id before all swap entries have
> dropped their references with mem_cgroup_uncharge_swap() ?  I think we
> only want to call __mem_cgroup_free() once all non kernel page
> references have been released.  This would include mem_cgroup_destroy()
> and any pending calls to mem_cgroup_uncharge_swap().  I'm not sure, but
> may be a second refcount or some optimization with the kmem_accounted
> bitmask can efficiently handle this.
> 

Can we demonstrate that? I agree there might be a potential problem, and
that is why I sent this separately. But the impression I got after
testing and reading the code, was that the memcg information in
pc->mem_cgroup would be updated to the parent.

This means that any later call to uncharge or uncharge_swap would just
uncharge from the parent memcg and we'd have no problem.

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