Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

[Michal Hocko]

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