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

[Ying Huang]

On Wed, 2022-06-08 at 13:58 +0530, Aneesh Kumar K V wrote:
> On 6/8/22 12:56 PM, Ying Huang wrote:
> > On Mon, 2022-06-06 at 14:03 +0530, Aneesh Kumar K V wrote:
> > > 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);
> > > }
> > > 
> > 
> > This should work.  But it appears unnatural.  So, I don't think we
> > should avoid to add more and more node masks to mitigate the design
> > decision that we cannot access memory tier information directly.  All
> > these becomes simple and natural, if we can access memory tier
> > information directly.
> > 
> 
> how do you derive whether node is toptier details if we have memtier 
> details in pgdat?

pgdat -> memory tier -> rank

Then we can compare this rank with the fast memory rank.  The fast
memory rank can be calculated dynamically at appropriate places.

Best Regards,
Huang, Ying