mm/vmpressure: reduce CPU, memory and code overhead on cgroup v2

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
 
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            | 303 +++---------------------------------
 4 files changed, 364 insertions(+), 292 deletions(-)
 create mode 100644 mm/vmpressure-v1.c

-- 
2.52.0