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

[Tim Chen]

On Thu, 2022-05-19 at 15:42 +0800, ying.huang@xxxxxxxxx wrote:
> 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.

Yes, while testing cgroup demotion, I also have to revert
the commit in question.
 
Acked-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> 
> 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;
> >  
> > 
> > 
> > 
> 
>