Re: [PATCH 4/4] memcg: always enable kmemcg on the default hierarchy

[Michal Hocko <mhocko@kernel.org>]

On Mon 07-09-15 13:01:10, Vladimir Davydov wrote:
> On Mon, Sep 07, 2015 at 11:39:06AM +0200, Michal Hocko wrote:
> ...
> > > > I might be wrong here of course but if the default should be switched it
> > > > would deserve a better justification with some numbers so that people
> > > > can see the possible drawbacks.
> > > 
> > > Personally, I'd prefer to have it switched on by default, because it
> > > would force people test it and report bugs and performance degradation.
> > > If one finds it really crappy, he/she should be able to disable it.
> > 
> > I do not think this is the way of introducing new functionality. You do
> > not want to force users to debug your code and go let it disable if it
> > is too crappy.
> 
> One must first enable CONFIG_KMEM, which is off by default.

Yes, but let's be realistic here. People tend to use distribution
kernels and so the config option will have to be enabled.
 
> Anyway, we aren't talking about enabling it by default in the legacy
> hierarchy, only in the unified hierarchy, which must be explicitly
> enabled by passing __DEVEL__save_behavior. I think that's enough.

But once it is set like that in default then it will stay even when the
__DEVEL__ part is dropped. 
 
> > > > I agree that the per-cgroup knob is better than the global one. We
> > > 
> > > Not that sure :-/
> > 
> > Why?
> 
> I'm not sure there is use cases which need having kmem acct enabled in
> one cgroup and disabled in another.

What would be the downside though? It is true that all the static keys
will be enabled in these configurations so even the non-enabled path
would pay some overhead but that should be still close to unmeasurable
(I haven't measured that so I might be wrong here).
-- 
Michal Hocko
SUSE Labs

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