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Date:   Wed, 7 Dec 2022 13:51:08 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] mm: memcontrol: deprecate charge moving
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On Tue, 6 Dec 2022 16:03:54 -0800 Shakeel Butt <shakeelb@google.com> wrote:

> 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.

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.