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

On Mon 17-08-26 21:58:39, Song Hu wrote:
> 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.

This would also allow more effective use of the stock because it
wouldn't need to throw other stock away when refilling.

-- 
Michal Hocko
SUSE Labs