Re: [PATCH] Revert "mm/vmscan: never demote for memcg reclaim"

[Yang Shi <shy828301@gmail.com>]

On Wed, May 18, 2022 at 12:09 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> This reverts commit 3a235693d3930e1276c8d9cc0ca5807ef292cf0a.
>
> Its premise was that cgroup reclaim cares about freeing memory inside
> the cgroup, and demotion just moves them around within the cgroup
> limit. Hence, pages from toptier nodes should be reclaimed directly.

Yes, exactly.

>
> However, with NUMA balancing now doing tier promotions, demotion is
> part of the page aging process. Global reclaim demotes the coldest
> toptier pages to secondary memory, where their life continues and from
> which they have a chance to get promoted back. Essentially, tiered
> memory systems have an LRU order that spans multiple nodes.
>
> When cgroup reclaims pages coming off the toptier directly, there can
> be colder pages on lower tier nodes that were demoted by global
> reclaim. This is an aging inversion, not unlike if cgroups were to
> reclaim directly from the active lists while there are inactive pages.

Thanks for pointing this out, makes sense to me.

>
> Proactive reclaim is another factor. The goal of that it is to offload
> colder pages from expensive RAM to cheaper storage. When lower tier
> memory is available as an intermediate layer, we want offloading to
> take advantage of it instead of bypassing to storage.
>
> Revert the patch so that cgroups respect the LRU order spanning the
> memory hierarchy.
>
> Of note is a specific undercommit scenario, where all cgroup limits in
> the system add up to <= available toptier memory. In that case,
> shuffling pages out to lower tiers first to reclaim them from there is
> inefficient. This is something could be optimized/short-circuited
> later on (although care must be taken not to accidentally recreate the
> aging inversion). Let's ensure correctness first.

Some side effects we might keep an eye with this revert:
  - Limit reclaim may experience longer latency since it has to do
demotion + reclaim to uncharge enough memory
  - Higher max usage due to the force charge from migration (of course
other migrations, i.e. NUMA fault, could have similar effect, but
anyway one more contributing factor)

They may not be noticeable hopefully, but I tend to agree that keeping
aging correct may be more important.

Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Roman Gushchin <guro@fb.com>
> ---
>  mm/vmscan.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c6918fff06e1..7a4090712177 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -528,13 +528,8 @@ static bool can_demote(int nid, struct scan_control *sc)
>  {
>         if (!numa_demotion_enabled)
>                 return false;
> -       if (sc) {
> -               if (sc->no_demotion)
> -                       return false;
> -               /* It is pointless to do demotion in memcg reclaim */
> -               if (cgroup_reclaim(sc))
> -                       return false;
> -       }
> +       if (sc && sc->no_demotion)
> +               return false;
>         if (next_demotion_node(nid) == NUMA_NO_NODE)
>                 return false;
>
> --
> 2.36.1
>
>