mm/vmpressure: skip tree=true accounting on cgroup v2

Patch series "mm/vmpressure: reduce CPU, memory and code overhead on
cgroup v2", v3.

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/memcontrol-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/memcontrol-v1.c is the whole subsystem.


This patch (of 2):

vmpressure() has two outputs gated by the @tree argument:

  @tree=false drives in-kernel socket pressure (mem_cgroup_set_
              socket_pressure), consumed by TCP/SCTP. This only
              applies on cgroup v2; on v1 socket memory is charged
              separately via tcpmem and the consumer reads
              memcg->tcpmem_pressure instead.

  @tree=true  drives userspace eventfd notifications via the v1
              memory.pressure_level / cgroup.event_control interface.
              v2 has no equivalent: userspace gets reclaim signals
              through memory.pressure (PSI), which does not touch
              vmpressure.

The existing early return covered v1 + @tree=false.  The symmetric v2 +
@tree=true case was falling through and doing the full lock / accumulate /
schedule_work / parent-walk dance for an events list that can never be
populated.  bpftrace on a 176-core production host (cgroup v2,
CONFIG_MEMCG_V1=n, 285 memcgs, sustained reclaim) showed ~16,200
@tree=true vmpressure() calls per minute.  Add an early return that skips
cgroup v2 + tree = true which avoids us doing all this work.  On a v2-only
host this also eliminates a lock contention path that can serialise
reclaimers on a single global sr_lock.

[usama.arif@linux.dev: simplify the guard]
  Link: https://lore.kernel.org/e8e1a409-48d8-4fa7-ae98-49485a1607f6@linux.dev
Link: https://lore.kernel.org/20260630112617.1198623-1-usama.arif@linux.dev
Link: https://lore.kernel.org/20260630112617.1198623-2-usama.arif@linux.dev
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions