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

[Michal Hocko <mhocko@kernel.org>]

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.
-- 
Michal Hocko
SUSE Labs