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

[Harry Yoo <harry@kernel.org>]

On 5/20/26 5:11 AM, Shakeel Butt wrote:
> On Wed, May 20, 2026 at 12:00:16AM +0900, Harry Yoo wrote:
>>
>>
> [...]
>>>
>>> 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?
> 
> The issue with that revert is that we reintroduce all node lru locking in the
> objcg reparenting path.

I'm not sure the problems with all-node locking are serious enough to 
rule it out as an option for 7.1.

It is not ideal, but given that the critical section for reparenting is 
independent of folio count, would this actually be a significant problem 
in practice? (even large servers rarely go beyond 8 NUMA nodes...)

>> 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).
> 
> I am waiting for Oliver to test the multi-objcg patch I sent. If that also
> resolves the regression then we have one more option i.e. backport that to 7.1
> to fix the regression.

Yeah, If per-node objcg is required in future kernels anyway, this 
option would be more ideal (if available).

> So to summarize, for future kernels we will be having multi-objcg in some form.
> For 7.1, we have to decide between (1) do nothing (2) this patch or (3) backport
> the multi-objcg path to 7.1.

Ack.

Thanks!

-- 
Cheers,
Harry / Hyeonggon