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Date:   Thu, 20 Jul 2023 17:02:22 -0700
From:   Roman Gushchin <roman.gushchin@linux.dev>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Yu Zhao <yuzhao@google.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        "T.J. Mercier" <tjmercier@google.com>,
        Greg Thelen <gthelen@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, cgroups@vger.kernel.org
Subject: Re: [RFC PATCH 0/8] memory recharging for offline memcgs
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References: <20230720070825.992023-1-yosryahmed@google.com>
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On Thu, Jul 20, 2023 at 07:08:17AM +0000, Yosry Ahmed wrote:
> This patch series implements the proposal in LSF/MM/BPF 2023 conference
> for reducing offline/zombie memcgs by memory recharging [1]. The main
> difference is that this series focuses on recharging and does not
> include eviction of any memory charged to offline memcgs.
> 
> Two methods of recharging are proposed:
> 
> (a) Recharging of mapped folios.
> 
> When a memcg is offlined, queue an asynchronous worker that will walk
> the lruvec of the offline memcg and try to recharge any mapped folios to
> the memcg of one of the processes mapping the folio. The main assumption
> is that a process mapping the folio is the "rightful" owner of the
> memory.
> 
> Currently, this is only supported for evictable folios, as the
> unevictable lru is imaginary and we cannot iterate the folios on it. A
> separate proposal [2] was made to revive the unevictable lru, which
> would allow recharging of unevictable folios.
> 
> (b) Deferred recharging of folios.
> 
> For folios that are unmapped, or mapped but we fail to recharge them
> with (a), we rely on deferred recharging. Simply put, any time a folio
> is accessed or dirtied by a userspace process, and that folio is charged
> to an offline memcg, we will try to recharge it to the memcg of the
> process accessing the folio. Again, we assume this process should be the
> "rightful" owner of the memory. This is also done asynchronously to avoid
> slowing down the data access path.

Unfortunately I have to agree with Johannes, Tejun and others who are not big
fans of this approach.

Lazy recharging leads to an interesting phenomena: a memory usage of a running
workload may suddenly go up only because some other workload is terminated and
now it's memory is being recharged. I find it confusing. It also makes hard
to set up limits and/or guarantees.

In general, I don't think we can handle shared memory well without getting rid
of "whoever allocates a page, pays the full price" policy and making a shared
ownership a fully supported concept. Of course, it's a huge work and I believe
the only way we can achieve it is to compromise on the granularity of the
accounting. Will the resulting system be better in the real life, it's hard to
say in advance.

Thanks!