Re: [PATCH v6 2/2] mm: add swapiness= arg to memory.reclaim

[Yu Zhao]

On Wed, Jan 3, 2024 at 9:49 AM Dan Schatzberg <schatzberg.dan@xxxxxxxxx> 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@xxxxxxxxx>
> Suggested-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Acked-by: Chris Li <chrisl@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 18 ++++----
>  include/linux/swap.h                    |  3 +-
>  mm/memcontrol.c                         | 56 ++++++++++++++++++++-----
>  mm/vmscan.c                             | 25 +++++++++--
>  4 files changed, 80 insertions(+), 22 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 3f85254f3cef..ee42f74e0765 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1282,17 +1282,10 @@ 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.
> -
>         Example::
>
>           echo "1G" > memory.reclaim
>
> -       The interface can be later extended with nested keys to
> -       configure the reclaim behavior. For example, specify the
> -       type of memory to reclaim from (anon, file, ..).
> -
>         Please note that the kernel can over or under reclaim from
>         the target cgroup. If less bytes are reclaimed than the
>         specified amount, -EAGAIN is returned.
> @@ -1304,6 +1297,17 @@ PAGE_SIZE multiple when read back.
>         This means that the networking layer will not adapt based on
>         reclaim induced by memory.reclaim.
>
> +The following nested keys are defined.
> +
> +         ==========            ================================
> +         swappiness            Swappiness value to reclaim with
> +         ==========            ================================
> +
> +       Specifying a swappiness value instructs the kernel to perform
> +       the reclaim with that swappiness value. Note that this has the
> +       same semantics as vm.swappiness applied to memcg reclaim with
> +       all the existing limitations and potential future extensions.
> +
>    memory.peak
>         A read-only single value file which exists on non-root
>         cgroups.
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index e2ab76c25b4a..8afdec40efe3 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -412,7 +412,8 @@ extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>                                                   unsigned long nr_pages,
>                                                   gfp_t gfp_mask,
> -                                                 unsigned int reclaim_options);
> +                                                 unsigned int reclaim_options,
> +                                                 int *swappiness);
>  extern unsigned long mem_cgroup_shrink_node(struct mem_cgroup *mem,
>                                                 gfp_t gfp_mask, bool noswap,
>                                                 pg_data_t *pgdat,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fbe9f02dd206..6d627a754851 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -52,6 +52,7 @@
>  #include <linux/sort.h>
>  #include <linux/fs.h>
>  #include <linux/seq_file.h>
> +#include <linux/parser.h>
>  #include <linux/vmpressure.h>
>  #include <linux/memremap.h>
>  #include <linux/mm_inline.h>
> @@ -2449,7 +2450,8 @@ static unsigned long reclaim_high(struct mem_cgroup *memcg,
>                 psi_memstall_enter(&pflags);
>                 nr_reclaimed += try_to_free_mem_cgroup_pages(memcg, nr_pages,
>                                                         gfp_mask,
> -                                                       MEMCG_RECLAIM_MAY_SWAP);
> +                                                       MEMCG_RECLAIM_MAY_SWAP,
> +                                                       NULL);
>                 psi_memstall_leave(&pflags);
>         } while ((memcg = parent_mem_cgroup(memcg)) &&
>                  !mem_cgroup_is_root(memcg));
> @@ -2740,7 +2742,7 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>
>         psi_memstall_enter(&pflags);
>         nr_reclaimed = try_to_free_mem_cgroup_pages(mem_over_limit, nr_pages,
> -                                                   gfp_mask, reclaim_options);
> +                                                   gfp_mask, reclaim_options, NULL);
>         psi_memstall_leave(&pflags);
>
>         if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
> @@ -3660,7 +3662,7 @@ static int mem_cgroup_resize_max(struct mem_cgroup *memcg,
>                 }
>
>                 if (!try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL,
> -                                       memsw ? 0 : MEMCG_RECLAIM_MAY_SWAP)) {
> +                                       memsw ? 0 : MEMCG_RECLAIM_MAY_SWAP, NULL)) {
>                         ret = -EBUSY;
>                         break;
>                 }
> @@ -3774,7 +3776,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
>                         return -EINTR;
>
>                 if (!try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL,
> -                                                 MEMCG_RECLAIM_MAY_SWAP))
> +                                                 MEMCG_RECLAIM_MAY_SWAP, NULL))
>                         nr_retries--;
>         }
>
> @@ -6720,7 +6722,7 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>                 }
>
>                 reclaimed = try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
> -                                       GFP_KERNEL, MEMCG_RECLAIM_MAY_SWAP);
> +                                       GFP_KERNEL, MEMCG_RECLAIM_MAY_SWAP, NULL);
>
>                 if (!reclaimed && !nr_retries--)
>                         break;
> @@ -6769,7 +6771,7 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>
>                 if (nr_reclaims) {
>                         if (!try_to_free_mem_cgroup_pages(memcg, nr_pages - max,
> -                                       GFP_KERNEL, MEMCG_RECLAIM_MAY_SWAP))
> +                                       GFP_KERNEL, MEMCG_RECLAIM_MAY_SWAP, NULL))
>                                 nr_reclaims--;
>                         continue;
>                 }
> @@ -6895,19 +6897,50 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
>         return nbytes;
>  }
>
> +enum {
> +       MEMORY_RECLAIM_SWAPPINESS = 0,
> +       MEMORY_RECLAIM_NULL,
> +};
> +
> +static const match_table_t tokens = {
> +       { MEMORY_RECLAIM_SWAPPINESS, "swappiness=%d"},
> +       { MEMORY_RECLAIM_NULL, NULL },
> +};
> +
>  static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
>                               size_t nbytes, loff_t off)
>  {
>         struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
>         unsigned int nr_retries = MAX_RECLAIM_RETRIES;
>         unsigned long nr_to_reclaim, nr_reclaimed = 0;
> +       int swappiness = -1;
>         unsigned int reclaim_options;
> -       int err;
> +       char *old_buf, *start;
> +       substring_t args[MAX_OPT_ARGS];
>
>         buf = strstrip(buf);
> -       err = page_counter_memparse(buf, "", &nr_to_reclaim);
> -       if (err)
> -               return err;
> +
> +       old_buf = buf;
> +       nr_to_reclaim = memparse(buf, &buf) / PAGE_SIZE;
> +       if (buf == old_buf)
> +               return -EINVAL;
> +
> +       buf = strstrip(buf);
> +
> +       while ((start = strsep(&buf, " ")) != NULL) {
> +               if (!strlen(start))
> +                       continue;
> +               switch (match_token(start, tokens, args)) {
> +               case MEMORY_RECLAIM_SWAPPINESS:
> +                       if (match_int(&args[0], &swappiness))
> +                               return -EINVAL;
> +                       if (swappiness < MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
> +                               return -EINVAL;
> +                       break;
> +               default:
> +                       return -EINVAL;
> +               }
> +       }
>
>         reclaim_options = MEMCG_RECLAIM_MAY_SWAP | MEMCG_RECLAIM_PROACTIVE;
>         while (nr_reclaimed < nr_to_reclaim) {
> @@ -6926,7 +6959,8 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
>
>                 reclaimed = try_to_free_mem_cgroup_pages(memcg,
>                                         min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
> -                                       GFP_KERNEL, reclaim_options);
> +                                       GFP_KERNEL, reclaim_options,
> +                                       swappiness == -1 ? NULL : &swappiness);
>
>                 if (!reclaimed && !nr_retries--)
>                         return -EAGAIN;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d91963e2d47f..394e0dd46b2e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -92,6 +92,11 @@ struct scan_control {
>         unsigned long   anon_cost;
>         unsigned long   file_cost;
>
> +#ifdef CONFIG_MEMCG
> +       /* Swappiness value for proactive reclaim. Always use sc_swappiness()! */
> +       int *proactive_swappiness;
> +#endif

Why is proactive_swappiness still a pointer? The whole point of the
previous conversation is that sc->proactive can tell whether
sc->swappiness is valid or not, and that's less awkward than using a
pointer.

Also why the #ifdef here? I don't see the point for a small stack
variable. Otherwise wouldn't we want to do this for sc->proactive as
well?

If you really want it to be explicit, you could do
  struct scan_control {
    ...
    struct {
      bool is_set;
      int swappiness;
    } proactive;
  };

But I think even this is too much.