On Wed 26-10-22 16:12:25, Aneesh Kumar K V wrote: > On 10/26/22 2:49 PM, 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. > > > > I was looking at this recently and I am wondering whether we should worry about VM_SHARE > vmas. > > ie, page_to_policy() can just reverse lookup just one VMA and fetch the policy right? How would that help for private mappings shared between parent/child? Also reducing this to a single VMA is not really necessary as folio_check_references already does most of that work. What is really missing is to check for other memory policies (i.e. cpusets and per-task mempolicy). The later is what can get quite expensive. > if it VM_SHARE it will be a shared policy we can find using vma->vm_file? > > For non anonymous and anon vma not having any policy set it is owning task vma->vm_mm->owner task policy ? Please note that mm can be shared even outside of the traditional thread group so you would need to go into something like mm_update_next_owner > We don't worry about multiple tasks that can be possibly sharing that page right? Why not? > > 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? > > I guess vma policy is important. Applications want to make sure that they don't > have variable performance and they go to lengths to avoid that by using MEM_BIND. > So if they access the memory they always know access is satisfied from a specific > set of NUMA nodes. Swapin can cause performance impact but then all continued > access will be from a specific NUMA nodes. With slow memory demotion that is > not going to be the case. Large in-memory database applications are observed to > be sensitive to these access latencies. Yes, I do understand that from the correctness POV this is a problem. My question is whether this is a _practical_ problem worth really being fixed as it is not really a cheap fix. If there are strong node locality assumptions by the userspace then I would expect demotion to be disabled in the first place. -- Michal Hocko SUSE Labs
