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

[Feng Tang]

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;

		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
> >
>