Re: [PATCH] memcg: multi-memcg percpu charge cache

Shakeel Butt
2025-04-23T22:17:07Z

Hi Jakub,

On Tue, Apr 22, 2025 at 06:10:22PM -0700, Jakub Kicinski wrote:
> On Wed, 16 Apr 2025 11:02:29 -0700 Shakeel Butt wrote:
> > static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> > {
> > struct memcg_stock_pcp *stock;
> > - unsigned int stock_pages;
> > + struct mem_cgroup *cached;
> > + uint8_t stock_pages;
>
> Is it okay to use uintX_t now?
>
> > unsigned long flags;
> > + bool evict = true;
> > + int i;
> >
> > VM_WARN_ON_ONCE(mem_cgroup_is_root(memcg));
> >
> > - if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> > + if (nr_pages > MEMCG_CHARGE_BATCH ||
> > + !local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> > /*
> > - * In case of unlikely failure to lock percpu stock_lock
> > - * uncharge memcg directly.
> > + * In case of larger than batch refill or unlikely failure to
> > + * lock the percpu stock_lock, uncharge memcg directly.
> > */
>
> We're bypassing the cache for > CHARGE_BATCH because the u8 math
> may overflow? Could be useful to refocus the comment on the 'why'
>

We actually never put more than MEMCG_CHARGE_BATCH in the cache and thus
we can use u8 as type here. Though we may increase the batch size in
future, so I should put a BUILD_BUG_ON somewhere here.

> > memcg_uncharge(memcg, nr_pages);
> > return;
> > }
>
> nits notwithstanding:
>
> Acked-by: Jakub Kicinski <kuba@kernel.org>

Thanks a lot for the review.