Re: [PATCH v2] memcg: first step towards hierarchical controller

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

> 
>> +	  of the root memcg, regardless of their positioning in the tree.
>> +
>> +	  Use of flat hierarchies is highly discouraged, but has been the
>> +	  default for performance reasons for quite some time. Setting this flag
>> +	  to on will make hierarchical accounting the default. It is still
>> +	  possible to set it back to flat by writing 0 to the file
>> +	  memory.use_hierarchy, albeit discouraged. Distributors are encouraged
>> +	  to set this option.
> [...]
> 
> I don't think that 'default n' is effective encouragement!
> 
> Ben.
> 
If it were up to me, I would just flip it to 1. No option.
A bit of history here, is that people have a - quite valid - concern
that this will disrupt users using their own kernel, should they decide
to update, recompile and run.

Conditional on a Kconfig option, people reusing their .config will see
no change. Distros, otoh, are versioned. It is not unreasonable to
expect a behavior change when a major version flips.

The encouragement here comes not from the default, but from the
acknowledgment that his thing is totally broken, and we need to act to
fix it in a compatible way.