On Wed, Oct 26, 2022 at 8:59 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Wed 26-10-22 20:20:01, Feng Tang wrote: > > 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. > > CONFIG_MEMCG=y is not sufficient. You would need to enable memcg > controller during the runtime as well. > > > > 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. > > Yes, it is definitely better to respect cpusets and all local memory > policies. There is no dispute there. The thing is whether this is really > worth it. How often would cpusets (or policies in general) go actively > against demotion nodes (i.e. exclude those nodes from their allowes node > mask)? > > I can imagine workloads which wouldn't like to get their memory demoted > for some reason but wouldn't it be more practical to tell that > explicitly (e.g. via prctl) rather than configuring cpusets/memory > policies explicitly? > > > 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. > > memcg is not the expensive part of the thing. You need to get from page > -> all vmas::vm_policy -> mm -> task::mempolicy Yeah, on the same page with Michal. Figuring out mempolicy from page seems quite expensive and the correctness can't be guranteed since the mempolicy could be set per-thread and the mm->task depends on CONFIG_MEMCG so it doesn't work for !CONFIG_MEMCG. > > -- > Michal Hocko > SUSE Labs >
