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

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

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?

-aneesh