Re: [PATCH v5 9/9] mm/demotion: Update node_is_toptier to work with memory tiers

[Aneesh Kumar K V]

On 6/6/22 12:54 PM, Ying Huang wrote:
> On Mon, 2022-06-06 at 09:22 +0530, Aneesh Kumar K V wrote:
> > On 6/6/22 8:41 AM, Ying Huang wrote:
> > > On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
> > > > With memory tiers support we can have memory on NUMA nodes
> > > > in the top tier from which we want to avoid promotion tracking NUMA
> > > > faults. Update node_is_toptier to work with memory tiers. To
> > > > avoid taking locks, a nodemask is maintained for all demotion
> > > > targets. All NUMA nodes are by default top tier nodes and as
> > > > we add new lower memory tiers NUMA nodes get added to the
> > > > demotion targets thereby moving them out of the top tier.
> > >
> > > Check the usage of node_is_toptier(),
> > >
> > > - migrate_misplaced_page()
> > >     node_is_toptier() is used to check whether migration is a promotion.
> > > We can avoid to use it.  Just compare the rank of the nodes.
> > >
> > > - change_pte_range() and change_huge_pmd()
> > >     node_is_toptier() is used to avoid scanning fast memory (DRAM) pages
> > > for promotion.  So I think we should change the name to node_is_fast()
> > > as follows,
> > >
> > > static inline bool node_is_fast(int node)
> > > {
> > > 	return NODE_DATA(node)->mt_rank >= MEMORY_RANK_DRAM;
> > > }
> >
> > But that gives special meaning to MEMORY_RANK_DRAM. As detailed in other
> > patches, absolute value of rank doesn't carry any meaning. It is only
> > the relative value w.r.t other memory tiers that decide whether it is
> > fast or not. Agreed by default memory tiers get built with
> > MEMORY_RANK_DRAM. But userspace can change the rank value of 'memtier1'
> > Hence to determine a node is consisting of fast memory is essentially
> > figuring out whether node is the top most tier in memory hierarchy and
> > not just the memory tier rank value is >= MEMORY_RANK_DRAM?
>
> In a system with 3 tiers,
>
> HBM	0
> DRAM	1
> PMEM	2
>
> In your implementation, only HBM will be considered fast.  But what we
> need is to consider both HBM and DRAM fast.  Because we use NUMA
> balancing to promote PMEM pages to DRAM.  It's unnecessary to scan HBM
> and DRAM pages for that.  And there're no requirements to promote DRAM
> pages to HBM with NUMA balancing.
>
> I can understand that the memory tiers are more dynamic now.  For
> requirements of NUMA balancing, we need the lowest memory tier (rank)
> where there's at least one node with CPU.  The nodes in it and the
> higher tiers will be considered fast.

is this good (not tested)?
/*
 * build the allowed promotion mask. Promotion is allowed
 * from higher memory tier to lower memory tier only if
 * lower memory tier doesn't include compute. We want to
 * skip promotion from a memory tier, if any node which is
 * part of that memory tier have CPUs. Once we detect such
 * a memory tier, we consider that tier as top tier from
 * which promotion is not allowed.
 */
list_for_each_entry_reverse(memtier, &memory_tiers, list) {
	nodes_and(allowed, node_state[N_CPU], memtier->nodelist);
	if (nodes_empty(allowed))
		nodes_or(promotion_mask, promotion_mask, allowed);
	else
		break;
}

and then

static inline bool node_is_toptier(int node)
{

	return !node_isset(node, promotion_mask);
}