Re: [PATCH v2] memcg: trim the per-cpu charge stock instead of draining it

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

On Thu, Aug 20, 2026 at 09:06:18AM +0200, Michal Hocko wrote:
> On Wed 19-08-26 18:20:10, Shakeel Butt wrote:
> > Joy reported that an application generating a request/response traffic
> > pattern spends 44.6% to 57.0% of CPU in the memcg charge/uncharge path
> > for a range of message sizes, against 0.27% to 0.71% outside that range.
> > Running from the root memcg, where socket memory accounting is skipped,
> > recovers the performance.
> > 
> > Tracing the charge path showed that the application generates a pattern
> > where the write syscall charges one page and the read syscall uncharges
> > two pages on the same CPU. This hits a corner case in the memcg percpu
> > stock code that thrashes the stock continuously.
> > 
> > In the memcg percpu stock code, MEMCG_CHARGE_BATCH (64) is both the high
> > watermark and the emptying target, i.e. on a request to charge one page
> > the kernel charges MEMCG_CHARGE_BATCH pages and caches
> > (MEMCG_CHARGE_BATCH - 1) of them in the percpu stock. The following
> > uncharge of 2 pages takes the cached count to (MEMCG_CHARGE_BATCH + 1),
> > and refill_stock() then empties the cache completely. With such a
> > pattern the percpu stock becomes completely ineffective.
> > 
> > Instead of a single boundary point for charges, use the technique the
> > page allocator uses for its own percpu caches, which keeps the watermark
> > and the emptying target apart: nr_pcp_free() frees between batch and
> > high - batch pages, leaving at least pcp->batch on the list. Add a high
> > watermark MEMCG_STOCK_HIGH and, once the cached count goes over it,
> > return only the pages above MEMCG_STOCK_LOW. The watermarks are
> > MEMCG_CHARGE_BATCH apart, so a page_counter update still covers a full
> > batch.
> > 
> > For now, keep MEMCG_STOCK_HIGH same as MEMCG_CHARGE_BATCH and in
> > future we will reevaluate if it makes sense to increase it.
> > 
> > Reported-by: Joy Chaoyue Xiong <cxiong@meta.com>
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks.

> One minor nit
> 
> > ---
> > 
> > Changes since v1:
> > http://lore.kernel.org/20260817234651.666540-1-shakeel.butt@linux.dev
> > - Kept the upper limit of memcg stock same as before (Michal)
> > 
> >  mm/memcontrol.c | 25 +++++++++++++++++++------
> >  1 file changed, 19 insertions(+), 6 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 17da1f43b7d3..58e4d23cf5e0 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> [...]
> > @@ -2254,9 +2264,12 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> >  			empty_slot = i;
> >  		if (memcg == READ_ONCE(stock->cached[i])) {
> >  			stock_pages = READ_ONCE(stock->nr_pages[i]) + nr_pages;
> > +			if (stock_pages > MEMCG_STOCK_HIGH) {
> > +				memcg_uncharge(memcg,
> > +					       stock_pages - MEMCG_STOCK_LOW);
> > +				stock_pages = MEMCG_STOCK_LOW;
> 
> I would find it easier to read to keep the update in sync with
> memcg_uncharge, i.e.
> 				stock_pages = WRITE_ONCE(stock_pages - MEMCG_STOCK_LOW)

Sorry I am not sure I understand your suggestion. The WRITE_ONCE() is throwing
me off.

> > +			}
> >  			WRITE_ONCE(stock->nr_pages[i], stock_pages);
> > -			if (stock_pages > MEMCG_CHARGE_BATCH)
> > -				drain_stock(stock, i);
> >  			success = true;
> >  			break;
> >  		}
> > -- 
> > 2.53.0-Meta
> 
> -- 
> Michal Hocko
> SUSE Labs