On Tue, Dec 6, 2022 at 9:14 AM Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxxx> Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx> 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.
