[PATCH v2 0/2] mm/vmpressure: reduce CPU, memory and code overhead on cgroup v2

[Usama Arif]

The vmpressure subsystem has two distinct consumers, gated by the
@tree argument:

  tree=false : in-kernel socket pressure, consumed by TCP/SCTP. This
               is cgroup v2 only; v1 sockets read memcg->tcpmem_pressure
               instead.
  tree=true  : cgroup v1 userspace eventfd notifications via the
               memory.pressure_level / cgroup.event_control interface.
               v2 has no equivalent (userspace gets reclaim signals
               through memory.pressure / PSI, which doesn't touch
               vmpressure).

So of the four (hierarchy, tree) combinations, only two carry data
that anyone reads. The existing early return in vmpressure() covered
v1 + tree=false; the symmetric v2 + tree=true case was falling through
and doing the full lock / accumulate / schedule_work / parent-walk
dance, even though the events list it eventually iterates is empty
on cgroup v2 (vmpressure_register_event() is wired up only through the
v1 cftype "memory.pressure_level" and can't be reached from a v2
memcg).

Patch 1 extends the existing early return to also skip v2 + tree=true.
On a v2-only host this eliminates a contended path where reclaimers
can serialize on a single global sr_lock. bpftrace on a 176-core production
host (cgroup v2, 285 memcgs, sustained reclaim) showed ~16,200 such calls
per minute with tree = true.

Patch 2 follows up with a cleanup: it splits the v1 userspace eventfd
interface (struct vmpressure_event, the events list and its mutex, the
work_struct and its handler, the parent walk,
vmpressure_register_event / unregister_event, and vmpressure_prio)
into a new mm/vmpressure-v1.c built only when CONFIG_MEMCG_V1=y,
behind small no-op stubs in the header. mm/vmpressure.c keeps the
shared bits and the tree=false socket-pressure path. The size of
vmpressure.c goes down to half and the code is much more simpler.
The only #ifdef CONFIG_MEMCG_V1 remaining in source is around the
v1-only fields inside struct vmpressure itself. Memory savings on
CONFIG_MEMCG_V1=n:
  struct vmpressure :  112B  ->  24B
  struct mem_cgroup : 1664B  -> 1536B

This split is the first step toward eventually making vmpressure
CONFIG_MEMCG_V1 only. The v2 in-kernel socket pressure path
(tree=false) cannot be removed today immediately: PSI is not an
exact replacement for vmpressure, and switching networking socket-buffer
back-off to PSI may regress networking performance or increase memory
pressure in workloads that today rely on vmpressure's hysteresis. The
medium-term plan is to introduce a PSI-based socket-pressure path, keep
vmpressure available for v2 behind a defconfig as an opt-out for several
releases, and only then drop the tree=false path entirely, at which point
everything that remains in mm/vmpressure-v1.c is the whole subsystem.
---
v1 -> v2: https://lore.kernel.org/all/20260606114158.3126210-1-usama.arif@xxxxxxxxx/
- Add more in commit message about future plans of vmpressure for cgroup v2
  (Shakeel)
- Remove unnecessary return statement in vmpressure for v1 only tree path
  (Michal)
- Rebased onto latest mm-new
 
Usama Arif (2):
  mm/vmpressure: skip tree=true accounting on cgroup v2
  mm/vmpressure: split v1 userspace eventfd code into vmpressure-v1.c

 include/linux/vmpressure.h |  46 +++++-
 mm/Makefile                |   2 +-
 mm/vmpressure-v1.c         | 305 +++++++++++++++++++++++++++++++++++++
 mm/vmpressure.c            | 302 ++----------------------------------
 4 files changed, 363 insertions(+), 292 deletions(-)
 create mode 100644 mm/vmpressure-v1.c

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2.53.0-Meta