[RFC] A question about memcg/kmem

[Vladimir Davydov <vdavydov@parallels.com>]

Hi,

There's one thing about kmemcg implementation that's bothering me. It's
about arrays holding per-memcg data (e.g. kmem_cache->memcg_params->
memcg_caches). On kmalloc or list_lru_{add,del} we want to quickly
lookup the copy of kmem_cache or list_lru corresponding to the current
cgroup. Currently, we hold all per-memcg caches/lists in an array
indexed by mem_cgroup->kmemcg_id. This allows us to lookup quickly, and
that's nice, but the arrays can grow indefinitely, because we reserve
slots for all cgroups, including offlined, and this is disastrous and
must be fixed.

I see several ways how to sort this out, but none of them looks perfect
to me, so I can't decide which one to choose. I would appreciate if you
could share your thoughts on them. Here they are:

1. When we are about to grow arrays (new kmem-active memcg is created
   and there's no slot for it), try to reclaim memory from all offline
   kmem-active cgroups in the hope one of them will pass away and
   release its slot.

   This is not very reliable obviously, because we can fail to reclaim
   and have to grow arrays anyway.

2. On css offline, empty all list_lru's corresponding to the dying
   cgroup by moving items to the parent. Then, we could free kmemcg_id
   immediately on offline, and the arrays would store entries for online
   cgroups only, which is fine. This looks as a kind of reparenting, but
   it doesn't move charges, only list_lru elements, which is much easier
   to do.

   This does not conform to how we treat other charges though.

3. Use some reclaimable data structure instead of a raw array. E.g.
   radix tree, or idr. The structure would grow then, but it would also
   shrink when css's are reclaimed on memory pressure.

   This will probably affect performance, because we do lookups on each
   kmalloc, so it must be as fast as possible. It could be probably
   optimized by caching the result of the last lookup (hint), but hints
   must be per cpu then, which will make list_lru bulky.

Currently, I incline to #1 or (most preferably) #2. I implemented
per-memcg list_lru with this in mind, and I have patches bringing in
list_lru "reparenting". #3 popped up in my mind just a few days ago. If
we decide to give it a try, I'll have to drop the previous per-memcg
list_lru implementation, and do a heavy rework of per-memcg kmem_cache
handling as well, but I'm fine with it.

I would be happy if we could opt out some of those design decisions
above. E.g. "I really hate #X, it's a no-go, because..." :-) Otherwise,
I'll most probably go with #2, which may become a nasty surprise to some
of you.

Thanks,
Vladimir

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