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

[ying.huang@xxxxxxxxx]

On Wed, 2022-05-18 at 15:09 -0400, Johannes Weiner 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.
> 
> 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.
> 
> 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.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

This is also required by Tim's DRAM partition among cgroups in tiered
sytstem.

Best Regards,
Huang, Ying

> ---
>  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;
>  
> 
> 
>