Michal Hocko <mhocko@xxxxxxxx> writes: > 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? If my understanding were correct, prctl() configures the process or thread. How can we get process/thread configuration at demotion time? Best Regards, Huang, Ying
