Re: [PATCH] mm: memcg/slab: optimize objcg stock draining

[Shakeel Butt <shakeelb@google.com>]

On Tue, Jan 5, 2021 at 8:22 PM Roman Gushchin <guro@fb.com> wrote:
>
> Imran Khan reported a regression in hackbench results caused by the
> commit f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects
> instead of pages"). The regression is noticeable in the case of
> a consequent allocation of several relatively large slab objects,
> e.g. skb's. As soon as the amount of stocked bytes exceeds PAGE_SIZE,
> drain_obj_stock() and __memcg_kmem_uncharge() are called, and it leads
> to a number of atomic operations in page_counter_uncharge().
>
> The corresponding call graph is below (provided by Imran Khan):
>   |__alloc_skb
>   |    |
>   |    |__kmalloc_reserve.isra.61
>   |    |    |
>   |    |    |__kmalloc_node_track_caller
>   |    |    |    |
>   |    |    |    |slab_pre_alloc_hook.constprop.88
>   |    |    |     obj_cgroup_charge
>   |    |    |    |    |
>   |    |    |    |    |__memcg_kmem_charge
>   |    |    |    |    |    |
>   |    |    |    |    |    |page_counter_try_charge
>   |    |    |    |    |
>   |    |    |    |    |refill_obj_stock
>   |    |    |    |    |    |
>   |    |    |    |    |    |drain_obj_stock.isra.68
>   |    |    |    |    |    |    |
>   |    |    |    |    |    |    |__memcg_kmem_uncharge
>   |    |    |    |    |    |    |    |
>   |    |    |    |    |    |    |    |page_counter_uncharge
>   |    |    |    |    |    |    |    |    |
>   |    |    |    |    |    |    |    |    |page_counter_cancel
>   |    |    |    |
>   |    |    |    |
>   |    |    |    |__slab_alloc
>   |    |    |    |    |
>   |    |    |    |    |___slab_alloc
>   |    |    |    |    |
>   |    |    |    |slab_post_alloc_hook
>
> Instead of directly uncharging the accounted kernel memory, it's
> possible to refill the generic page-sized per-cpu stock instead.
> It's a much faster operation, especially on a default hierarchy.
> As a bonus, __memcg_kmem_uncharge_page() will also get faster,
> so the freeing of page-sized kernel allocations (e.g. large kmallocs)
> will become faster.
>
> A similar change has been done earlier for the socket memory by
> the commit 475d0487a2ad ("mm: memcontrol: use per-cpu stocks for
> socket memory uncharging").
>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Reported-by: Imran Khan <imran.f.khan@oracle.com>

I remember seeing this somewhere
https://lore.kernel.org/linux-mm/20190423154405.259178-1-shakeelb@google.com/

Reviewed-by: Shakeel Butt <shakeelb@google.com>