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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Sat, Jun 06, 2026 at 04:41:32AM -0700, Usama Arif wrote:
> 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.

We should really move v2 away from vmpressure.

>   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.

This is good.

> 
> 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

For this, I am wondering if we should just go ahead and work towards making
vmpressure memcg-v1 only unless we foresee a lot of or complex work is needed
for that and only then patch 2 makes sense.