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

[Huang, Ying]

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