[RFC PATCH v5 0/3] Add memory.max.effective for application's allocators

Michal Koutný
2024-06-06T15:22:46Z

Some applications use memory cgroup limits to scale their own memory
needs. Reading of the immediate membership cgroup's memory.max is not
sufficient because of possible ancestral limits. The application could
traverse upwards to figure out the tightest limit but this would not
work in cgroup namespace where the view of cgroup hierarchy is
incomplete and the limit may apply from outer world.
Additionally, applications should respond to limit changes.

(cgroup v1 used memory.stat:hierarchical_memory_limit to report the
value but there's no such counterpart in cgroup v2 memory.stat.)

Introduce a new memcg attribute file that contains the effective value
of memory limit for given cgroup (following cpuset.cpus.effective
pattern) and that sends notifications like memory.events when the
effective limit changes.

Reasons for RFC:
1) Should global limit be included? (And respond to memory hotplug?)
2) Is swap.max.effective needed? (in v2 without memsw accounting)
3) Should memory.high be also handled?
4) What would be an alternative?

My answers to RFC:

1) No (there's no memory.max in global root memcg)
2) No (app doesn't have full control of memory that's swapped out)
3) No (scaling the allocator against the "soft" limit could end up in
dynamics difficult to reason and admin)
4)
- PSI (too obscure for traditional users but better semantics for limit
shrinking)
- memory.stat field (like v1 but separate attribute is better for
notifications, cpuset precedent)

Changes from v4 (https://lore.kernel.org/r/ZcvlhOZ4VBEX9raZ@host1.jankratochvil.net)
- split the patch for swap.max.effetive
- add Documentation/
- reword commit messages
- add notification support

Michal Koutný (3):
memcg: Add memory.max.effective attribute
memcg: Add memory.swap.max.effective like hierarchical_memsw_limit
memcg: Notify on memory.max.effective changes

Documentation/admin-guide/cgroup-v2.rst | 6 ++++
include/linux/memcontrol.h | 2 ++
mm/memcontrol.c | 46 +++++++++++++++++++++++++
3 files changed, 54 insertions(+)

base-commit: 2df0193e62cf887f373995fb8a91068562784adc
--
2.45.1