Re: [PATCH 3/3] mm: memcontrol: deprecate charge moving

[Shakeel Butt <shakeelb@google.com>]

On Tue, Dec 6, 2022 at 9:14 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> Charge moving mode in cgroup1 allows memory to follow tasks as they
> migrate between cgroups. This is, and always has been, a questionable
> thing to do - for several reasons.
>
> First, it's expensive. Pages need to be identified, locked and
> isolated from various MM operations, and reassigned, one by one.
>
> Second, it's unreliable. Once pages are charged to a cgroup, there
> isn't always a clear owner task anymore. Cache isn't moved at all, for
> example. Mapped memory is moved - but if trylocking or isolating a
> page fails, it's arbitrarily left behind. Frequent moving between
> domains may leave a task's memory scattered all over the place.
>
> Third, it isn't really needed. Launcher tasks can kick off workload
> tasks directly in their target cgroup. Using dedicated per-workload
> groups allows fine-grained policy adjustments - no need to move tasks
> and their physical pages between control domains. The feature was
> never forward-ported to cgroup2, and it hasn't been missed.
>
> Despite it being a niche usecase, the maintenance overhead of
> supporting it is enormous. Because pages are moved while they are live
> and subject to various MM operations, the synchronization rules are
> complicated. There are lock_page_memcg() in MM and FS code, which
> non-cgroup people don't understand. In some cases we've been able to
> shift code and cgroup API calls around such that we can rely on native
> locking as much as possible. But that's fragile, and sometimes we need
> to hold MM locks for longer than we otherwise would (pte lock e.g.).
>
> Mark the feature deprecated. Hopefully we can remove it soon.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

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

I would request this patch to be backported to stable kernels as well
for early warnings to users which update to newer kernels very late.