Re: [PATCH -V6 07/14] memcg: Add HugeTLB extension

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Thu, May 24, 2012 at 02:52:26PM -0700, David Rientjes wrote:
> On Mon, 16 Apr 2012, Aneesh Kumar K.V wrote:
> 
> > This patch implements a memcg extension that allows us to control HugeTLB
> > allocations via memory controller. The extension allows to limit the
> > HugeTLB usage per control group and enforces the controller limit during
> > page fault. Since HugeTLB doesn't support page reclaim, enforcing the limit
> > at page fault time implies that, the application will get SIGBUS signal if it
> > tries to access HugeTLB pages beyond its limit. This requires the application
> > to know beforehand how much HugeTLB pages it would require for its use.
> > 
> > The charge/uncharge calls will be added to HugeTLB code in later patch.
> > Support for memcg removal will be added in later patches.
> > 
> 
> Again, I disagree with this approach because it's adding the functionality 
> to memcg when it's unnecessary; it would be a complete legitimate usecase 
> to want to limit the number of globally available hugepages to a set of 
> tasks without incurring the per-page tracking from memcg.
> 
> This can be implemented as a seperate cgroup and as we move to a single 
> hierarchy, you lose no functionality if you mount both cgroups from what 
> is done here.
> 
> It would be much cleaner in terms of
> 
>  - build: not requiring ifdefs and dependencies on CONFIG_HUGETLB_PAGE, 
>    which is a prerequisite for this functionality and is not for 
>    CONFIG_CGROUP_MEM_RES_CTLR,

I am not sure we have large number of #ifdef as you have outlined above.
Most of the hugetlb limit code is well isolated already. If we were to
split it as a seperate controller, we will be duplicating code related
cgroup deletion,  migration support etc from memcg, because in case
of memcg and hugetlb limit they depend on struct page. So I would expect
we would be end up #ifdef around that code or duplicate them in the
new controller if we were to do hugetlb limit as a seperate controller.

Another reason for it to be part of memcg is, it is normal to look
at hugetlb usage also as a memory usage. One of the feedback I got
for the earlier post is to see if i can enhace the current code to
make sure memory.usage_in_bytes can also account for hugetlb usage.
People would also like to look at memory.limit_in_bytes to limit total
usage. (inclusive of hugetlb).

> 
>  - code: seperating hugetlb bits out from memcg bits to avoid growing 
>    mm/memcontrol.c beyond its current 5650 lines, and
> 

I can definitely look at spliting mm/memcontrol.c 


>  - performance: not incurring any overhead of enabling memcg for per-
>    page tracking that is unnecessary if users only want to limit hugetlb 
>    pages.
> 

-aneesh