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

[Johannes Weiner]

On Tue, Dec 06, 2022 at 05:58:14PM -0800, Hugh Dickins wrote:
> On Tue, 6 Dec 2022, Johannes Weiner 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: Hugh Dickins <hughd@xxxxxxxxxx>

Thanks

> but I wonder if it would be helpful to mention move_charge_at_immigrate
> in the deprecation message: maybe the first line should be
> "Cgroup memory moving (move_charge_at_immigrate) is deprecated.\n"

Fair enough! Here is the updated patch.

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