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SubjectRe: [PATCH -V6 07/14] memcg: Add HugeTLB extension
Date
"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> writes:

> 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.
>>

Since Andrew didn't sent the patchset to Linus because of this
discussion, I looked at reworking the patchset as a seperate
controller. The patchset I sent here

http://thread.gmane.org/gmane.linux.kernel.mm/79230

have seen minimal testing. I also folded the fixup patches
Andrew had in -mm to original patchset.

Let me know if the changes looks good.
-aneesh



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