Messages in this thread | | | Date | Wed, 8 Jun 2022 13:58:10 +0530 | Subject | Re: [PATCH v5 9/9] mm/demotion: Update node_is_toptier to work with memory tiers | From | Aneesh Kumar K V <> |
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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
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