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

[Michal Koutný]

On Tue, Jan 05, 2021 at 08:22:39PM -0800, 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   <--- draining old memcg
>   |    |    |    |    |    |    |
>   |    |    |    |    |    |    |__memcg_kmem_uncharge
>   |    |    |    |    |    |    |    |
>   |    |    |    |    |    |    |    |page_counter_uncharge
>   |    |    |    |    |    |    |    |    |
>   |    |    |    |    |    |    |    |    |page_counter_cancel
> [...]
> -	page_counter_uncharge(&memcg->memory, nr_pages);
> -	if (do_memsw_account())
> -		page_counter_uncharge(&memcg->memsw, nr_pages);
> +	refill_stock(memcg, nr_pages);
I noticed that refill_stock(memcg,...) switches the local stock to
memcg.

In this particular call chain, the uncharged memcg is the old memcg of
stock->cached_objcg. The refill_stock() then may switch stock->cached to
the old memcg too. If the patch leads to better performance, then the
switch probably doesn't happen at this moment (and I guess
stock->cached_objcg and stock->cached can be independent to some extent,
so the old memcg in one needn't be the old in the latter).

In conclusion
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>


Michal

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