On Tue, 6 Dec 2022 16:03:54 -0800 Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > 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. Sounds reasonable, but the changelog should have a few words in it explaining why we're requesting the backport. I guess I can type those in. We're at -rc8 and I'm not planning on merging these up until after 6.2-rc1 is out. Please feel free to argue with me on that score.
