On Mon, Aug 31, 2026 at 09:37:47AM -0700, Joshua Hahn wrote:
> In order to avoid expensive hierarchy walks on every memcg charge and
> limit check, memcontrol uses per-cpu stocks (memcg_stock_pcp) to cache
> pre-charged pages and introduce a fast path to try_charge_memcg.
>
> However, there are a few quirks with the current implementation that
> can be improved upon.
>
> First, each memcg_stock_pcp can only cache the charges of 7 memcgs
> (NR_MEMCG_STOCK). When an 8th memcg wants to cache its charge on a CPU,
> a victim memcg is chosen among the 7 cached memcgs and is evicted,
> losing all cached charges.
>
> Second, stock draining is per-CPU rather than per-memcg. That is,
> when a memcg is under pressure and must retrieve all cached charges,
> it iterates through every CPU and drains the stock charges of all
> present memcgs. This means that one under-pressure memcg evicts the
> caches of all co-cpu-resident memcg stock caches.
>
> Finally, stock is tightly coupled with memcg, so adding new
> page_counters to memcg is an unscalable operation where only one counter
> gets to use the fastpath.
>
> We can address all of these concerns by pushing stock caches down to the
> page_counter level, and making each counter responsible for its own
> charge.
>
> Introduce struct page_counter_stock along with its allocation, free, and
> per-CPU drain helpers.
>
> No functional change intended.
>
> Suggested-by: Johannes Weiner <[email protected]>
> Signed-off-by: Joshua Hahn <[email protected]>
> ---
> include/linux/page_counter.h | 16 +++++++
> mm/page_counter.c | 90 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 106 insertions(+)
>
> diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> index 89a083f16fbf7..c1fe331f34e7e 100644
> --- a/include/linux/page_counter.h
> +++ b/include/linux/page_counter.h
> @@ -5,8 +5,11 @@
> #include <linux/atomic.h>
> #include <linux/cache.h>
> #include <linux/limits.h>
> +#include <linux/workqueue_types.h>
> #include <asm/page.h>
>
> +struct page_counter_stock;
> +
> struct page_counter {
> /*
> * Make sure 'usage' does not share cacheline with any other field in
> @@ -41,6 +44,13 @@ struct page_counter {
> unsigned long high;
> unsigned long max;
> struct page_counter *parent;
> + struct page_counter_stock __percpu *stock;
On gcc 14.2.1, I get
In file included from mm/page_counter.c:8:
./include/linux/page_counter.h:47:44: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or
‘__attribute__’ before ‘*’ token
47 | struct page_counter_stock __percpu *stock;
| ^
I think you need an appropriate header in this file.
