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

[Yang Shi]

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.

>
>                 if (task) {
>                         mpol = get_task_policy(task);
>                         if (mpol)
>                                 mpol_get(mpol);
>                 }
>         }
>
>         if (!mpol)
>                 return ret;
>
>         if (mpol->mode != MPOL_BIND)
>                 goto put_exit;
>
>         nid = folio_nid(folio);
>         dnid = next_demotion_node(nid);
>         if (!node_isset(dnid, mpol->nodes)) {
>                 *skip_demotion = true;
>                 ret = false;
>         }
>
> put_exit:
>         mpol_put(mpol);
>         return ret;
> }
>
> static unsigned int shrink_page_list(struct list_head *page_list,..)
> {
>         ...
>
>         bool skip_demotion = false;
>         struct rmap_walk_control rwc = {
>                 .arg = &skip_demotion,
>                 .rmap_one = __check_mpol_demotion,
>         };
>
>         /* memory policy check */
>         rmap_walk(folio, &rwc);
>         if (skip_demotion)
>                 goto keep_locked;
> }
>
> And there seems to be no simple solution for getting the memory
> policy from a page.
>
> Thanks,
> Feng
>
> > >
> > > --
> > > Michal Hocko
> > > SUSE Labs
> > >
> >