[Devel] Re: [PATCH v2 5/5] decrement static keys on real destroy time

[Glauber Costa]

On 04/23/2012 11:40 PM, KAMEZAWA Hiroyuki wrote:
> (2012/04/24 4:37), Glauber Costa wrote:
> 
>> We call the destroy function when a cgroup starts to be removed,
>> such as by a rmdir event.
>>
>> However, because of our reference counters, some objects are still
>> inflight. Right now, we are decrementing the static_keys at destroy()
>> time, meaning that if we get rid of the last static_key reference,
>> some objects will still have charges, but the code to properly
>> uncharge them won't be run.
>>
>> This becomes a problem specially if it is ever enabled again, because
>> now new charges will be added to the staled charges making keeping
>> it pretty much impossible.
>>
>> We just need to be careful with the static branch activation:
>> since there is no particular preferred order of their activation,
>> we need to make sure that we only start using it after all
>> call sites are active. This is achieved by having a per-memcg
>> flag that is only updated after static_key_slow_inc() returns.
>> At this time, we are sure all sites are active.
>>
>> This is made per-memcg, not global, for a reason:
>> it also has the effect of making socket accounting more
>> consistent. The first memcg to be limited will trigger static_key()
>> activation, therefore, accounting. But all the others will then be
>> accounted no matter what. After this patch, only limited memcgs
>> will have its sockets accounted.
>>
>> [v2: changed a tcp limited flag for a generic proto limited flag ]
>> [v3: update the current active flag only after the static_key update ]
>>
>> Signed-off-by: Glauber Costa<glommer at parallels.com>
> 
> 
> Acked-by: KAMEZAWA Hiroyuki<kamezawa.hiroyu at jp.fujitsu.com>
> 
> A small request below.
> 
> <snip>
> 
> 
>> +		 * ->activated needs to be written after the static_key update.
>> +		 *  This is what guarantees that the socket activation function
>> +		 *  is the last one to run. See sock_update_memcg() for details,
>> +		 *  and note that we don't mark any socket as belonging to this
>> +		 *  memcg until that flag is up.
>> +		 *
>> +		 *  We need to do this, because static_keys will span multiple
>> +		 *  sites, but we can't control their order. If we mark a socket
>> +		 *  as accounted, but the accounting functions are not patched in
>> +		 *  yet, we'll lose accounting.
>> +		 *
>> +		 *  We never race with the readers in sock_update_memcg(), because
>> +		 *  when this value change, the code to process it is not patched in
>> +		 *  yet.
>> +		 */
>> +		mutex_lock(&tcp_set_limit_mutex);
> 
> 
> Could you explain for what this mutex is in above comment ?
> 
This is explained at the site where the mutex is defined.
If you still want me to mention it here, or maybe expand the explanation
there, I surely can.