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

[Feng Tang]

On Thu, Oct 27, 2022 at 03:45:12PM +0800, Huang, Ying wrote:
> Feng Tang <feng.tang@xxxxxxxxx> writes:
> 
> > 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;
> 
>                 task = NULL;
> 
> > 		if (vma->vm_mm)
> > 			task = vma->vm_mm->owner;
> >
> > 		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;
> > 	}
> 
> I think that you need to get a node mask instead.  Even if
> !node_isset(dnid, mpol->nodes), you may demote to other node in the node
> mask.

Yes, you are right. This code was written/tested about 2 months ago,
before Aneesh's memory tiering interface patchset. It was listed
to demonstrate idea of solution. 

Thanks,
Feng

> Best Regards,
> Huang, Ying