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

[Feng Tang]

On Thu, Oct 27, 2022 at 10:55:58AM -0700, Yang Shi wrote:
> On Thu, Oct 27, 2022 at 12:12 AM Feng Tang <feng.tang@xxxxxxxxx> wrote:
> >
> > On Thu, Oct 27, 2022 at 01:57:52AM +0800, Yang Shi wrote:
> > > On Wed, Oct 26, 2022 at 8:59 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > [...]
> > > > > > 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.
> >
> > Yes, you are right. Our "working" psudo code for mem policy looks like
> > what Michal mentioned, and it can't work for all cases, but try to
> > enforce it whenever possible:
> >
> > static bool  __check_mpol_demotion(struct folio *folio, struct vm_area_struct *vma,
> >                 unsigned long addr, void *arg)
> > {
> >         bool *skip_demotion = arg;
> >         struct mempolicy *mpol;
> >         int nid, dnid;
> >         bool ret = true;
> >
> >         mpol = __get_vma_policy(vma, addr);
> >         if (!mpol) {
> >                 struct task_struct *task;
> >                 if (vma->vm_mm)
> >                         task = vma->vm_mm->owner;
> 
> But this task may not be the task you want IIUC. For example, the
> process has two threads, A and B. They have different mempolicy. The
> vmscan is trying to demote a page belonging to thread A, but the task
> may point to thread B, so you actually get the wrong mempolicy IIUC.

Yes, this is a valid concern! We don't have good solution for this.
For memory policy, we may only handle the per-vma policy for now whose
cost is relatively low, as a best-effort try.

Thanks,
Feng