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

[Yang Shi]

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
>