Date: Thu, 21 Dec 2023 22:31:59 -0700
From: Yu Zhao <yuzhao@...gle.com>
To: Dan Schatzberg <schatzberg.dan@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Yosry Ahmed <yosryahmed@...gle.com>, Huan Yang <link@...o.com>, linux-kernel@...r.kernel.org, 
	cgroups@...r.kernel.org, linux-mm@...ck.org, Tejun Heo <tj@...nel.org>, 
	Zefan Li <lizefan.x@...edance.com>, Jonathan Corbet <corbet@....net>, 
	Michal Hocko <mhocko@...nel.org>, Shakeel Butt <shakeelb@...gle.com>, 
	Muchun Song <muchun.song@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, SeongJae Park <sj@...nel.org>, 
	"Vishal Moola (Oracle)" <vishal.moola@...il.com>, Nhat Pham <nphamcs@...il.com>, 
	Yue Zhao <findns94@...il.com>
Subject: Re: [PATCH v5 2/2] mm: add swapiness= arg to memory.reclaim

On Wed, Dec 20, 2023 at 8:27 AM Dan Schatzberg <schatzberg.dan@...il.com> wrote:
>
> Allow proactive reclaimers to submit an additional swappiness=<val>
> argument to memory.reclaim. This overrides the global or per-memcg
> swappiness setting for that reclaim attempt.
>
> For example:
>
> echo "2M swappiness=0" > /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.
>
> Userspace proactive reclaimers use the memory.reclaim interface to
> trigger reclaim. The memory.reclaim interface does not allow for any way
> to effect the balance of file vs anon during proactive reclaim. The only
> approach is to adjust the vm.swappiness setting. However, there are a
> few reasons we look to control the balance of file vs anon during
> proactive reclaim, separately from reactive reclaim:
>
> * Swapout should be limited to manage SSD write endurance. In near-OOM
> situations we are fine with lots of swap-out to avoid OOMs. As these are
> typically rare events, they have relatively little impact on write
> endurance. However, proactive reclaim runs continuously and so its
> impact on SSD write endurance is more significant. Therefore it is
> desireable to control swap-out for proactive reclaim separately from
> reactive reclaim
>
> * Some userspace OOM killers like systemd-oomd[1] support OOM killing on
> swap exhaustion. This makes sense if the swap exhaustion is triggered
> due to reactive reclaim but less so if it is triggered due to proactive
> reclaim (e.g. one could see OOMs when free memory is ample but anon is
> just particularly cold). Therefore, it's desireable to have proactive
> reclaim reduce or stop swap-out before the threshold at which OOM
> killing occurs.
>
> In the case of Meta's Senpai proactive reclaimer, we adjust
> vm.swappiness before writes to memory.reclaim[2]. This has been in
> production for nearly two years and has addressed our needs to control
> proactive vs reactive reclaim behavior but is still not ideal for a
> number of reasons:
>
> * vm.swappiness is a global setting, adjusting it can race/interfere
> with other system administration that wishes to control vm.swappiness.
> In our case, we need to disable Senpai before adjusting vm.swappiness.
>
> * vm.swappiness is stateful - so a crash or restart of Senpai can leave
> a misconfigured setting. This requires some additional management to
> record the "desired" setting and ensure Senpai always adjusts to it.
>
> With this patch, we avoid these downsides of adjusting vm.swappiness
> globally.
>
> [1]https://www.freedesktop.org/software/systemd/man/latest/systemd-oomd.service.html
> [2]https://github.com/facebookincubator/oomd/blob/main/src/oomd/plugins/Senpai.cpp#L585-L598
>
> Signed-off-by: Dan Schatzberg <schatzberg.dan@...il.com>

The cover letter says:
"Previously, this exact interface addition was proposed by Yosry[3]."

So I think it should be acknowledged with a Suggested-by, based on:
"A Suggested-by: tag indicates that the patch idea is suggested by the
person named and ensures credit to the person for the idea."
from
https://docs.kernel.org/process/submitting-patches.html#using-reported-by-tested-by-reviewed-by-suggested-by-and-fixes

>  Documentation/admin-guide/cgroup-v2.rst | 18 ++++----
>  include/linux/swap.h                    |  3 +-
>  mm/memcontrol.c                         | 56 ++++++++++++++++++++-----
>  mm/vmscan.c                             | 13 +++++-
>  4 files changed, 69 insertions(+), 21 deletions(-)

...

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d91963e2d47f..aa5666842c49 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -92,6 +92,9 @@ struct scan_control {
>         unsigned long   anon_cost;
>         unsigned long   file_cost;
>
> +       /* Swappiness value for reclaim. NULL will fall back to per-memcg/global value */
> +       int *swappiness;

Using a pointer to indicate whether the type it points to is
overridden isn't really a good practice.

A better alternative was suggested during the v2:
"Perhaps the negative to avoid unnecessary dereferences."
https://lore.kernel.org/linux-mm/dhhjw4h22q4ngwtxmhuyifv32zjd6z2relrcjgnxsw6zys3mod@o6dh5dy53ae3/

Since only proactive reclaim can override swappiness, meaning it only
happens if sc->proactive is true, I think the best way to make it work
without spending much effort is create a helper as Michal suggest but
it should look like:

sc_swappiness()
{
  return sc->proactive ? sc->swappiness : mem_cgroup_swappiness(memcg);
}

In this patchset, sc->swappiness really means
sc->proactive_swappiness. So it should be renamed accordingly.

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