Re: [PATCH RFC 3/4] mm: kmem: prepare remote memcg charging infra for interrupt contexts

[Shakeel Butt <shakeelb@google.com>]

On Thu, Aug 27, 2020 at 10:52 AM Roman Gushchin <guro@fb.com> wrote:
>
> Remote memcg charging API uses current->active_memcg to store the
> currently active memory cgroup, which overwrites the memory cgroup
> of the current process. It works well for normal contexts, but doesn't
> work for interrupt contexts: indeed, if an interrupt occurs during
> the execution of a section with an active memcg set, all allocations
> inside the interrupt will be charged to the active memcg set (given
> that we'll enable accounting for allocations from an interrupt
> context). But because the interrupt might have no relation to the
> active memcg set outside, it's obviously wrong from the accounting
> prospective.
>
> To resolve this problem, let's add a global percpu int_active_memcg
> variable, which will be used to store an active memory cgroup which
> will be sued from interrupt contexts. set_active_memcg() will

*used

> transparently use current->active_memcg or int_active_memcg depending
> on the context.
>
> To make the read part simple and transparent for the caller, let's
> introduce two new functions:
>   - struct mem_cgroup *active_memcg(void),
>   - struct mem_cgroup *get_active_memcg(void).
>
> They are returning the active memcg if it's set, hiding all
> implementation details: where to get it depending on the current context.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>

I like this patch. Internally we have a similar patch which instead of
per-cpu int_active_memcg have current->active_memcg_irq. Our use-case
was radix tree node allocations where we use the root node's memcg to
charge all the nodes of the tree and the reason behind was that we
observed a lot of zombies which were stuck due to radix tree nodes
charges while the actual pages pointed by the those nodes/entries were
in used by active jobs (shared file system and the kernel is older
than the kmem reparenting).

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