Re: [RFC PATCH] mm/mglru: keep the root_memcg reclaim behavior the same as memcg reclaim

[T.J. Mercier]

On Thu, Dec 12, 2024 at 1:57 AM hailong <hailong.liu@xxxxxxxx> wrote:
>
> From: Hailong Liu <hailong.liu@xxxxxxxx>
>
> commit a579086c99ed ("mm: multi-gen LRU: remove eviction fairness safeguard") said
> Note that memcg LRU only applies to global reclaim. For memcg reclaim,
> the eviction will continue, even if it is overshooting. This becomes
> unconditional due to code simplification.
>
> Howeven, if we reclaim a root memcg by sysfs (memory.reclaim), the behavior acts
> as a kswapd or direct reclaim.

Hi Hailong,

Why do you think this is a problem?

> Fix this by remove the condition of mem_cgroup_is_root in
> root_reclaim().
> Signed-off-by: Hailong Liu <hailong.liu@xxxxxxxx>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 76378bc257e3..1f74f3ba0999 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -216,7 +216,7 @@ static bool cgroup_reclaim(struct scan_control *sc)
>   */
>  static bool root_reclaim(struct scan_control *sc)
>  {
> -       return !sc->target_mem_cgroup || mem_cgroup_is_root(sc->target_mem_cgroup);
> +       return !sc->target_mem_cgroup;
>  }
>
>  /**
> --
> Actually we switch to mglru on kernel-6.1 and see different behavior on
> root_mem_cgroup reclaim. so is there any background fot this?

Reclaim behavior differs with MGLRU.
https://lore.kernel.org/lkml/20221201223923.873696-1-yuzhao@xxxxxxxxxx/

On even more recent kernels, regular LRU reclaim has also changed.
https://lore.kernel.org/lkml/20240514202641.2821494-1-hannes@xxxxxxxxxxx/

> Brs,
> Hailong.
>