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

Aneesh Kumar K V
2022-06-06T08:39:02Z

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);
}