Re: [PATCH v3] memcg: cache obj_stock by memcg, not by objcg pointer

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

On Tue, May 19, 2026 at 03:46:51PM +0900, Harry Yoo wrote:
> 
> 
> On 5/19/26 8:41 AM, Shakeel Butt wrote:
> > On Mon, May 18, 2026 at 03:28:27PM -0700, Shakeel Butt wrote:
> > > Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
> > > per-node type") split a memcg's single obj_cgroup into one per NUMA
> > > node, but the per-CPU obj_stock_pcp still keys cached_objcg by
> > > pointer. Cross-NUMA workloads now see a drain on every refill and a
> > > miss on every consume that targets a sibling per-node objcg of the
> > > same memcg, producing the 67.7% stress-ng switch-mq regression
> > > reported by LKP.
> > > 
> > > stock->nr_bytes are fungible across per-node objcgs of one memcg.
> > > Treat the cache as keyed by memcg in __consume_obj_stock() and
> > > __refill_obj_stock() so siblings share the reserve. Compare via
> > > READ_ONCE(objcg->memcg) directly: pointer-compare only, no deref, so
> > > the rcu_read_lock contract on obj_cgroup_memcg() does not apply.
> > > 
> > > Sharing the reserve without re-caching means bytes funded by one
> > > per-node objcg's slow path can be consumed/freed under a different
> > > sibling, leaving sub-page residue on whichever sibling was cached at
> > > drain time. The pre-existing obj_cgroup_release() path would WARN and
> > > silently drop that residue, leaking up to nr_node_ids * (PAGE_SIZE - 1)
> > > bytes per memcg lifecycle from the page_counter. Forward the residue
> > > into a per-node objcg of the same (post-reparent) memcg at release time
> > > instead, so it can be reconciled later via a refill atomic_xchg or
> > > another release; the chain terminates at root_mem_cgroup, whose
> > > page_counter has no enforced limit.
> > > 
> > > Please note that this is temporary fix and will be reverted when
> > > per-node kmem accounting is introduced.
> 
> ... because once per-node kmem accounting is introduced,
> "stock->nr_bytes are fungible across per-node objcgs of one memcg"
> no longer holds?

Yes

> 
> And the follow-up plain is to revert this and address it with a multi-objcg
> percpu stock [1], similar to a multi-memcg percpu charge cache we have now,
> right? (regardless of per-node kmem accounting's progress)
> 

Yes

> If this temporary fix imposes other potential correctness issues, would it
> make sense to land [1] in mainline before the next LTS release and skip this
> temporary fix?
> 
> [1] https://lore.kernel.org/oe-lkp/agtPMpQK2jXdQAY4@linux.dev
> 

The full clean solution might take one more cycle and I think we can not just
ignore 67% regression on 7.1.