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

[Muchun Song <songmuchun@bytedance.com>]

On Wed, May 18, 2022 at 03:09:11PM -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@cmpxchg.org>

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.