On Wed, Oct 26, 2022 at 05:19:50PM +0800, Michal Hocko wrote: > On Wed 26-10-22 16:00:13, Feng Tang wrote: > > On Wed, Oct 26, 2022 at 03:49:48PM +0800, Aneesh Kumar K V wrote: > > > On 10/26/22 1:13 PM, Feng Tang wrote: > > > > In page reclaim path, memory could be demoted from faster memory tier > > > > to slower memory tier. Currently, there is no check about cpuset's > > > > memory policy, that even if the target demotion node is not allowd > > > > by cpuset, the demotion will still happen, which breaks the cpuset > > > > semantics. > > > > > > > > So add cpuset policy check in the demotion path and skip demotion > > > > if the demotion targets are not allowed by cpuset. > > > > > > > > > > What about the vma policy or the task memory policy? Shouldn't we respect > > > those memory policy restrictions while demoting the page? > > > > Good question! We have some basic patches to consider memory policy > > in demotion path too, which are still under test, and will be posted > > soon. And the basic idea is similar to this patch. > > For that you need to consult each vma and it's owning task(s) and that > to me sounds like something to be done in folio_check_references. > Relying on memcg to get a cpuset cgroup is really ugly and not really > 100% correct. Memory controller might be disabled and then you do not > have your association anymore. You are right, for cpuset case, the solution depends on 'CONFIG_MEMCG=y', and the bright side is most of distribution have it on. > This all can get quite expensive so the primary question is, does the > existing behavior generates any real issues or is this more of an > correctness exercise? I mean it certainly is not great to demote to an > incompatible numa node but are there any reasonable configurations when > the demotion target node is explicitly excluded from memory > policy/cpuset? We haven't got customer report on this, but there are quite some customers use cpuset to bind some specific memory nodes to a docker (You've helped us solve a OOM issue in such cases), so I think it's practical to respect the cpuset semantics as much as we can. Your concern about the expensive cost makes sense! Some raw ideas are: * if the shrink_folio_list is called by kswapd, the folios come from the same per-memcg lruvec, so only one check is enough * if not from kswapd, like called form madvise or DAMON code, we can save a memcg cache, and if the next folio's memcg is same as the cache, we reuse its result. And due to the locality, the real check is rarely performed. Thanks, Feng > -- > Michal Hocko > SUSE Labs >
