Re: [PATCH 1/2] memcg: folding CONFIG_MEMCG_SWAP as default

[Alex Shi <alex.shi@linux.alibaba.com>]

在 2020/4/18 上午12:54, Michal Hocko 写道:
> On Fri 17-04-20 09:41:04, Shakeel Butt wrote:
>> On Fri, Apr 17, 2020 at 9:03 AM Michal Hocko <mhocko@kernel.org> wrote:
>>>
>>> On Fri 17-04-20 22:43:43, Alex Shi wrote:
>>>> This patch fold MEMCG_SWAP feature into kernel as default function. That
>>>> required a short size memcg id for each of page. As Johannes mentioned
>>>>
>>>> "the overhead of tracking is tiny - 512k per G of swap (0.04%).'
>>>>
>>>> So all swapout page could be tracked for its memcg id.
>>>
>>> I am perfectly OK with dropping the CONFIG_MEMCG_SWAP. The code that is
>>> guarded by it is negligible and the resulting code is much easier to
>>> read so no objection on that front. I just do not really see any real
>>> reason to flip the default for cgroup v1. Why do we want/need that?
>>>
>>
>> Yes, the changelog is lacking the motivation of this change. This is
>> proposed by Johannes and I was actually expecting the patch from him.
>> The motivation is to make the things simpler for per-memcg LRU locking
>> and workingset for anon memory (Johannes has described these really
>> well, lemme find the email). If we keep the differentiation between
>> cgroup v1 and v2, then there is actually no point of this cleanup as
>> per-memcg LRU locking and anon workingset still has to handle the
>> !do_swap_account case.
> 
> All those details really have to go into the changelog. I have to say
> that I still do not understand why the actual accounting swap or not
> makes any difference for per per-memcg LRU. Especially when your patch
> keeps the kernel command line parameter still in place.
> 
> Anyway, it would be much more simpler to have a patch that drops the
> CONFIG_MEMCG_SWAP and a separate one which switches the default
> beahvior. I am not saying I am ok with the later but if the
> justification is convincing then I might change my mind.
> 

Hi Shakeel & Michal,

Thanks for all comments!

Yes, we still need to remove swapaccount from cmdline and keep swap_cgroup.id
permanently. Just I don't know if this patch could fit into the details of 
Johannes new solution.

Anyway, I will send out v2 for complete memcg id record patch, just in case
if they are useful.

Thanks
Alex