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References: <20231211140419.1298178-1-schatzberg.dan@gmail.com> <20231211140419.1298178-2-schatzberg.dan@gmail.com>
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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Mon, 11 Dec 2023 11:41:24 -0800
Message-ID: <CAJD7tkZQ2aakT8M2bTg0bp4sDtrGYv_4i4Z4z3KBerfxZ9qFWA@mail.gmail.com>
Subject: Re: [PATCH V3 1/1] mm: add swapiness= arg to memory.reclaim
To:     Dan Schatzberg <schatzberg.dan@gmail.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Huan Yang <link@vivo.com>, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Chris Li <chrisl@kernel.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        "Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
        Yue Zhao <findns94@gmail.com>, Hugh Dickins <hughd@google.com>
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On Mon, Dec 11, 2023 at 6:04=E2=80=AFAM Dan Schatzberg <schatzberg.dan@gmai=
l.com> wrote:
>
> Allow proactive reclaimers to submit an additional swappiness=3D<val>
> argument to memory.reclaim. This overrides the global or per-memcg
> swappiness setting for that reclaim attempt.
>
> For example:
>
> echo "2M swappiness=3D0" > /sys/fs/cgroup/memory.reclaim
>
> will perform reclaim on the rootcg with a swappiness setting of 0 (no
> swap) regardless of the vm.swappiness sysctl setting.
>
> Signed-off-by: Dan Schatzberg <schatzberg.dan@gmail.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 15 ++++++-
>  include/linux/swap.h                    |  3 +-
>  mm/memcontrol.c                         | 55 ++++++++++++++++++++-----
>  mm/vmscan.c                             | 13 +++++-
>  4 files changed, 70 insertions(+), 16 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admi=
n-guide/cgroup-v2.rst
> index 3f85254f3cef..fc2b379dbd0f 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1282,8 +1282,8 @@ PAGE_SIZE multiple when read back.
>         This is a simple interface to trigger memory reclaim in the
>         target cgroup.
>
> -       This file accepts a single key, the number of bytes to reclaim.
> -       No nested keys are currently supported.
> +       This file accepts a string which containers thhe number of bytes

contains* the*

I think this statement was only important because no keys were
supported, so I think we can remove it completely and rely on
documenting the supported keys below like other interfaces, see my
next comment.

> +       to reclaim.
>
>         Example::
>
> @@ -1304,6 +1304,17 @@ PAGE_SIZE multiple when read back.
>         This means that the networking layer will not adapt based on
>         reclaim induced by memory.reclaim.
>
> +       This file also allows the user to specify the swappiness value
> +       to be used for the reclaim. For example:
> +
> +         echo "1G swappiness=3D60" > memory.reclaim
> +
> +       The above instructs the kernel to perform the reclaim with
> +       a swappiness value of 60. Note that this has the same semantics
> +       as the vm.swappiness sysctl - it sets the relative IO cost of
> +       reclaiming anon vs file memory but does not allow for reclaiming
> +       specific amounts of anon or file memory.
> +

Can we instead follow the same format used by other nested-keyed files
(e.g. io.max)? This usually involves a table of supported keys and
such.

>    memory.peak
>         A read-only single value file which exists on non-root
>         cgroups.
[..]
> @@ -6902,12 +6913,33 @@ static ssize_t memory_reclaim(struct kernfs_open_=
file *of, char *buf,
>         unsigned int nr_retries =3D MAX_RECLAIM_RETRIES;
>         unsigned long nr_to_reclaim, nr_reclaimed =3D 0;
>         unsigned int reclaim_options;
> -       int err;
> +       char *old_buf, *start;
> +       substring_t args[MAX_OPT_ARGS];
> +       int swappiness =3D -1;
>
>         buf =3D strstrip(buf);
> -       err =3D page_counter_memparse(buf, "", &nr_to_reclaim);
> -       if (err)
> -               return err;
> +
> +       old_buf =3D buf;
> +       nr_to_reclaim =3D memparse(buf, &buf) / PAGE_SIZE;
> +       if (buf =3D=3D old_buf)
> +               return -EINVAL;
> +
> +       buf =3D strstrip(buf);
> +
> +       while ((start =3D strsep(&buf, " ")) !=3D NULL) {
> +               if (!strlen(start))
> +                       continue;
> +               switch (match_token(start, if_tokens, args)) {
> +               case MEMORY_RECLAIM_SWAPPINESS:
> +                       if (match_int(&args[0], &swappiness))
> +                               return -EINVAL;
> +                       if (swappiness < 0 || swappiness > 200)

I am not a fan of extending the hardcoded 0 and 200 values, and now
the new -1 value. Maybe it's time to create constants for the min and
max swappiness values instead of hardcoding them everywhere? This can
be a separate preparatory patch. Then, -1 (or any invalid value) can
also be added as a constant with a useful name, instead of passing -1
to all other callers.

This should make the code a little bit more readable and easier to extend.