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

[Harry Yoo]

On 5/19/26 11:02 PM, Shakeel Butt wrote:
> 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

Thanks for confirming!

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

That is valid point, unfortunately.

One more thing I have to ask... for v7.1, wouldn't it be a safer option 
to revert the per-node object change and re-introduce it once we have a 
cleaner solution?

This change was introduced in v5, but the implementation before v4 had 
been exposed in -next for a while, and I think we don't have enough 
justification to keep the per-node objcgs change, at least for v7.1, 
given that we have an unexpected last-minute regression and
correctness concerns (albeit slight).

--
Cheers,
Harry / Hyeonggon