> Note: Assume a big system which has many cpus, and user wants to devide

the system into containers. Current memcg's percpu caching is done
only when a task in memcg is on the cpu, running. So, it's not so dangerous
as it looks.

Agree. I actually think it is pretty

But yes, if we can drop memcg's code, it's good. Then, we can remove some
amount of codes.

quoted

But the cons:

* percpu counters have signed quantities, so this would limit us 4G.
   We can add a shift and then count pages instead of bytes, but we
   are still in the 16T area here. Maybe we really need more than that.

....
struct percpu_counter {
         raw_spinlock_t lock;
         s64 count;

s64 limtes us 4G ?

Yes, I actually explicitly mentioned that. We can go to 16T if we track
pages
instead of bytes (I considered having the res_counter initialization code to
specify a shift, so we could be generic).

But I believe that if we go this route, we'll need to either:
1) Have our own internal implementation of what percpu counters does
2) create u64 acessors that would cast that to u64 in the operations.
Since it
     is a 64 bit field anyway it should be doable. But being doable
doesn't mean we
     should do it....
3) Have a different percpu_counter structure, something like struct
percpu_positive_counter.

quoted

* some of the additions here may slow down the percpu_counters for
   users that don't care about our usage. Things about min/max tracking
   enter in this category.

I think it's not very good to increase size of percpu counter. It's already
very big...Hm. How about

	struct percpu_counter_lazy {
		struct percpu_counter pcp;
		extra information
		s64 margin;
	}
?

Can work, but we need something that also solves the signedness problem.
Maybe we can use a union for that, and then stuff things in the end of a
different
structure just for the users that want it.

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