On Tue, Jan 5, 2021 at 8:22 PM Roman Gushchin <guro@xxxxxx> 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@xxxxxx>
> Reported-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
I remember seeing this somewhere
https://lore.kernel.org/linux-mm/20190423154405.259178-1-shakeelb@xxxxxxxxxx/
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
