Re: memcg: slab control

[Pavel Emelyanov <xemul@parallels.com>]

David Rientjes wrote:
> On Thu, 26 Nov 2009, Pavel Emelyanov wrote:
> 
>> I'm ready to resurrect the patches and port them for slab.
>> But before doing it we should answer one question.
>>
> 
> Do you have a pointer to your latest implementation that you proposed for 
> slab?

I believe this is the one:
https://lists.linux-foundation.org/pipermail/containers/2007-September/007481.html

>> Consider we have two kmalloc-s in a kernel code - one is
>> user-space triggerable and the other one is not. From my
>> POV we should account for the former one, but should not
>> for the latter.
>>
>> If so - how should we patch the kernel to achieve that goal?
>>
> 
> I think all slab allocations should be accounted for based on current's 
> memcg other than those done in hardirq context, annotating slab 
> allocations doesn't seem scalable.  Whether the accounting is done on a 
> task level or cgroup level isn't really a problem for us since we don't 
> move tasks amongst cgroups.  I imagine there've been previous restrictions 
> on that put into place with the memcg so this doesn't seem like a 
> slabcg-specific requirement anyway.
> 
> The problem on the freeing side is mapping the object back to the cgroup 
> that allocated it.  We'd also need to map the object to the context in 
> which it was allocated to determine whether we should decrement the 
> counter or not.  How do you propose doing that without a considerable 
> overhead in memory consumption, fastpath branch, and cache cold slabcg 
> lookups?

That's the biggest problem. Generally speaking - no other way rather than
store additional pointer. In some situations you can rely on the cgroup of
a task in which context an object is being freed, but in that case once you
move a task to another cgroup your accounting is screwed.

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