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Message-ID: <Z9l-x-b9W32vE8Qn@google.com>
Date: Tue, 18 Mar 2025 14:10:15 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Zhongkun He <hezhongkun.hzk@...edance.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, yuzhao@...gle.com,
mhocko@...e.com, muchun.song@...ux.dev, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add swappiness=max arg to memory.reclaim for only
anon reclaim
On Tue, Mar 18, 2025 at 09:53:30PM +0800, Zhongkun He wrote:
> With this patch 'commit <68cd9050d871> ("mm: add swappiness= arg to
> memory.reclaim")', we can submit an additional swappiness=<val> argument
> to memory.reclaim. It is very useful because we can dynamically adjust
> the reclamation ratio based on the anonymous folios and file folios of
> each cgroup. For example,when swappiness is set to 0, we only reclaim
> from file folios.
>
> However,we have also encountered a new issue: when swappiness is set to
> the MAX_SWAPPINESS, it may still only reclaim file folios.
>
> So, we hope to add a new arg 'swappiness=max' in memory.reclaim where
> proactive memory reclaim only reclaims from anonymous folios when
> swappiness is set to max. The swappiness semantics from a user
> perspective remain unchanged.
>
> For example, something like this:
>
> echo "2M swappiness=max" > /sys/fs/cgroup/memory.reclaim
>
> will perform reclaim on the rootcg with a swappiness setting of 'max' (a
> new mode) regardless of the file folios. Users have a more comprehensive
> view of the application's memory distribution because there are many
> metrics available. For example, if we find that a certain cgroup has a
> large number of inactive anon folios, we can reclaim only those and skip
> file folios, because with the zram/zswap, the IO tradeoff that
> cache_trim_mode or other file first logic is making doesn't hold -
> file refaults will cause IO, whereas anon decompression will not.
>
> With this patch, the swappiness argument of memory.reclaim has a new
> mode 'max', means reclaiming just from anonymous folios both in traditional
> LRU and MGLRU.
Is MGLRU handled in this patch?
>
> Here is the previous discussion:
> https://lore.kernel.org/all/20250314033350.1156370-1-hezhongkun.hzk@bytedance.com/
> https://lore.kernel.org/all/20250312094337.2296278-1-hezhongkun.hzk@bytedance.com/
>
> Suggested-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> Signed-off-by: Zhongkun He <hezhongkun.hzk@...edance.com>
> ---
> Documentation/admin-guide/cgroup-v2.rst | 4 ++++
> include/linux/swap.h | 4 ++++
> mm/memcontrol.c | 5 +++++
> mm/vmscan.c | 10 ++++++++++
> 4 files changed, 23 insertions(+)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index cb1b4e759b7e..c39ef4314499 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1343,6 +1343,10 @@ The following nested keys are defined.
> same semantics as vm.swappiness applied to memcg reclaim with
> all the existing limitations and potential future extensions.
>
> + If set swappiness=max, memory reclamation will exclusively
> + target the anonymous folio list for both traditional LRU and
> + MGLRU reclamation algorithms.
> +
I don't think we need to specify LRU and MGLRU here. What about:
Setting swappiness=max exclusively reclaims anonymous memory.
> memory.peak
> A read-write single value file which exists on non-root cgroups.
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index b13b72645db3..a94efac10fe5 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -419,6 +419,10 @@ extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> #define MEMCG_RECLAIM_PROACTIVE (1 << 2)
> #define MIN_SWAPPINESS 0
> #define MAX_SWAPPINESS 200
> +
> +/* Just recliam from anon folios in proactive memory reclaim */
> +#define ONLY_ANON_RECLAIM_MODE (MAX_SWAPPINESS + 1)
> +
This is a swappiness value so let's keep that clear, e.g.
SWAPPINESS_ANON_ONLY or similar.
> extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> unsigned long nr_pages,
> gfp_t gfp_mask,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4de6acb9b8ec..0d0400f141d1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4291,11 +4291,13 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
>
> enum {
> MEMORY_RECLAIM_SWAPPINESS = 0,
> + MEMORY_RECLAIM_ONLY_ANON_MODE,
> MEMORY_RECLAIM_NULL,
> };
>
> static const match_table_t tokens = {
> { MEMORY_RECLAIM_SWAPPINESS, "swappiness=%d"},
> + { MEMORY_RECLAIM_ONLY_ANON_MODE, "swappiness=max"},
MEMORY_RECLAIM_SWAPPINESS_MAX?
> { MEMORY_RECLAIM_NULL, NULL },
> };
>
> @@ -4329,6 +4331,9 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
> if (swappiness < MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
> return -EINVAL;
> break;
> + case MEMORY_RECLAIM_ONLY_ANON_MODE:
> + swappiness = ONLY_ANON_RECLAIM_MODE;
> + break;
> default:
> return -EINVAL;
> }
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c767d71c43d7..779a9a3cf715 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2438,6 +2438,16 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
> goto out;
> }
>
> + /*
> + * Do not bother scanning file folios if the memory reclaim
> + * invoked by userspace through memory.reclaim and set
> + * 'swappiness=max'.
> + */
/* Proactive reclaim initiated by userspace for anonymous memory only */
> + if (sc->proactive && (swappiness == ONLY_ANON_RECLAIM_MODE)) {
Do we need to check sc->proactive here? Supposedly this swappiness value
can only be passed in from proactive reclaim. Instead of silently
ignoring the value from other paths, I wonder if we should WARN on
!sc->proactive instead.
> + scan_balance = SCAN_ANON;
> + goto out;
> + }
> +
> /*
> * Do not apply any pressure balancing cleverness when the
> * system is close to OOM, scan both anon and file equally
> --
> 2.39.5
>
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