Re: [PATCH] mm: memcg: flush empty per-cpu stock slots on memcg offlining

[Song Hu]

Hi,Michal

在 2026/8/17 21:29, Michal Hocko 写道:
> On Mon 17-08-26 21:12:21, Song Hu wrote:
>> On Mon 17-08-26, Michal Hocko wrote:
>>> Is there any specific reason why the memcg stays in the cache slot
>>> without any pages?
>>
>> consume_stock() doesn't release the slot when nr_pages hits zero.  It
>> is kept for the next charge of the same task and only gets displaced
>> by a charge under a different memcg or by CPU hotplug.  The problem is
>> that the offlining drain skips empty slots, so the css reference they
>> hold is never dropped unless something unrelated displaces them.
> 
> This doesn't answer my question, really, does it? Is there any good
> reason for this implementation? Why do we need to drop references
> remotely when we can do so when the last cached charge is consumed?
> 

Fair enough.  There is no strong reason.  Keeping the slot
populated after the last page is consumed only saves a
css_get()/css_put() pair when the same memcg charges again on that
CPU - a micro-optimization from the original single-slot
implementation.

Dropping the reference in consume_stock() when the slot empties is
the better place.  Empty slots stop existing, so the offlining drain
has nothing left to miss and is_memcg_drain_needed() stays as it
is.  This also makes Joshua's concern about the full-stock drain go
away entirely.  The cost is one refcount pair per emptied slot, at
most once per MEMCG_CHARGE_BATCH pages.

Joshua, this supersedes the css_is_dying gating you suggested and
that I said I would do - with no empty slots left, the check would
never fire, and the kill_css_sync() ordering argument becomes moot
as well.  Since you are reworking this code, I'd appreciate a
sanity check on releasing from consume_stock().

If this direction works for you both, I'll rework the patch
accordingly.

Thanks,
Song