[Devel] Re: [PATCH v3 00/28] kmem limitation for memcg

Kamezawa Hiroyuki kamezawa.hiroyu at jp.fujitsu.com
Wed Jun 13 19:24:53 PDT 2012

(2012/06/07 23:00), Frederic Weisbecker wrote:
> On Thu, Jun 07, 2012 at 02:53:07PM +0400, Glauber Costa wrote:
>> On 06/07/2012 02:26 PM, Frederic Weisbecker wrote:
>>> On Fri, May 25, 2012 at 05:03:20PM +0400, Glauber Costa wrote:
>>>> Hello All,
>>>> This is my new take for the memcg kmem accounting. This should merge
>>>> all of the previous comments from you, plus fix a bunch of bugs.
>>>> At this point, I consider the series pretty mature. Since last submission
>>>> 2 weeks ago, I focused on broadening the testing coverage. Some bugs were
>>>> fixed, but that of course doesn't mean no bugs exist.
>>>> I believe some of the early patches here are already in some trees around.
>>>> I don't know who should pick this, so if everyone agrees with what's in here,
>>>> please just ack them and tell me which tree I should aim for (-mm? Hocko's?)
>>>> and I'll rebase it.
>>>> I should point out again that most, if not all, of the code in the caches
>>>> are wrapped in static_key areas, meaning they will be completely patched out
>>>> until the first limit is set. Enabling and disabling of static_keys incorporate
>>>> the last fixes for sock memcg, and should be pretty robust.
>>>> I also put a lot of effort, as you will all see, in the proper separation
>>>> of the patches, so the review process is made as easy as the complexity of
>>>> the work allows to.
>>> So I believe that if I want to implement a per kernel stack accounting/limitation,
>>> I need to work on top of your patchset.
>>> What do you think about having some sub kmem accounting based on the caches?
>>> For example there could be a specific accounting per kmem cache.
>>> Like if we use a specific kmem cache to allocate the kernel stack
>>> (as is done by some archs but I can generalize that for those who want
>>> kernel stack accounting), allocations are accounted globally in the memcg as
>>> done in your patchset but also on a seperate counter only for this kmem cache
>>> on the memcg, resulting in a kmem.stack.usage somewhere.
>>> The concept of per kmem cache accounting can be expanded more for any
>>> kind of finegrained kmem accounting.
>>> Thoughts?
>> I believe a general separation is too much, and will lead to knob
>> explosion. So I don't think it is a good idea.
> Right. This could be an option in kmem_cache_create() or something.
>> Now, for the stack itself, it can be justified. The question that
>> remains to be answered is:
>> Why do you need to set the stack value separately? Isn't accounting
>> the stack value, and limiting against the global kmem limit enough?
> Well, I may want to let my container have a full access to some kmem
> resources (net, file, etc...) but defend against fork bombs or NR_PROC
> rlimit exhaustion of other containers.
> So I need to be able to set my limit precisely on kstack.

You explained that the limitation is necessary for fork-bomb, and the bad
point of fork-bomb is that it can cause OOM. So, the problem is OOM not fork-bomb.

If the problem is OOM, IIUC, generic kernel memory limiting will work better than
kernel stack limiting.


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