[PATCH v3 0/5] mm: memcg accounting of percpu memory

[Roman Gushchin <guro@fb.com>]

This patchset adds percpu memory accounting to memory cgroups.
It's based on the rework of the slab controller and reuses concepts
and features introduced for the per-object slab accounting.

Percpu memory is becoming more and more widely used by various
subsystems, and the total amount of memory controlled by the percpu
allocator can make a good part of the total memory.

As an example, bpf maps can consume a lot of percpu memory,
and they are created by a user. Also, some cgroup internals
(e.g. memory controller statistics) can be quite large.
On a machine with many CPUs and big number of cgroups they
can consume hundreds of megabytes.

So the lack of memcg accounting is creating a breach in the memory
isolation. Similar to the slab memory, percpu memory should be
accounted by default.

Percpu allocations by their nature are scattered over multiple pages,
so they can't be tracked on the per-page basis. So the per-object
tracking introduced by the new slab controller is reused.

The patchset implements charging of percpu allocations, adds
memcg-level statistics, enables accounting for percpu allocations made
by memory cgroup internals and provides some basic tests.

To implement the accounting of percpu memory without a significant
memory and performance overhead the following approach is used:
all accounted allocations are placed into a separate percpu chunk
(or chunks). These chunks are similar to default chunks, except
that they do have an attached vector of pointers to obj_cgroup objects,
which is big enough to save a pointer for each allocated object.
On the allocation, if the allocation has to be accounted
(__GFP_ACCOUNT is passed, the allocating process belongs to a non-root
memory cgroup, etc), the memory cgroup is getting charged and if the maximum
limit is not exceeded the allocation is performed using a memcg-aware
chunk. Otherwise -ENOMEM is returned or the allocation is forced over
the limit, depending on gfp (as any other kernel memory allocation).
The memory cgroup information is saved in the obj_cgroup vector
at the corresponding offset. On the release time the memcg
information is restored from the vector and the cgroup is getting
uncharged.
Unaccounted allocations (at this point the absolute majority
of all percpu allocations) are performed in the old way, so no
additional overhead is expected.

To avoid pinning dying memory cgroups by outstanding allocations,
obj_cgroup API is used instead of directly saving memory cgroup pointers.
obj_cgroup is basically a pointer to a memory cgroup with a standalone
reference counter. The trick is that it can be atomically swapped
to point at the parent cgroup, so that the original memory cgroup
can be released prior to all objects, which has been charged to it.
Because all charges and statistics are fully recursive, it's perfectly
correct to uncharge the parent cgroup instead. This scheme is used
in the slab memory accounting, and percpu memory can just follow
the scheme.

v3:
  1) fixed a build errors and warning with !CONFIG_MEMCG_KMEM (Andrew)
  2) fixed a build warning on Clang (Nathan)
  3) rebased on top of v7 of the slab controller patchset

v2:
  1) minor cosmetic fixes (Dennis)
  2) rebased on top of v6 of the slab controller patchset

v1:
  1) fixed a bug with gfp flags handling (Dennis)
  2) added some comments (Tejun and Dennis)
  3) rebased on top of v5 of the slab controller patchset

RFC:
  https://lore.kernel.org/linux-mm/20200519201806.2308480-1-guro@fb.com/T/#t


Roman Gushchin (5):
  percpu: return number of released bytes from pcpu_free_area()
  mm: memcg/percpu: account percpu memory to memory cgroups
  mm: memcg/percpu: per-memcg percpu memory statistics
  mm: memcg: charge memcg percpu memory to the parent cgroup
  kselftests: cgroup: add perpcu memory accounting test

 Documentation/admin-guide/cgroup-v2.rst    |   4 +
 include/linux/memcontrol.h                 |   8 +
 mm/memcontrol.c                            |  18 +-
 mm/percpu-internal.h                       |  55 +++++-
 mm/percpu-km.c                             |   5 +-
 mm/percpu-stats.c                          |  36 ++--
 mm/percpu-vm.c                             |   5 +-
 mm/percpu.c                                | 208 ++++++++++++++++++---
 tools/testing/selftests/cgroup/test_kmem.c |  70 ++++++-
 9 files changed, 360 insertions(+), 49 deletions(-)

-- 
2.26.2