path: root/mm/internal.h
diff options
authorDave Hansen <>2021-09-02 14:59:06 -0700
committerLinus Torvalds <>2021-09-03 09:58:16 -0700
commit79c28a41672278283fa72e03d0bf80e6644d4ac4 (patch)
tree8ffc45ae86a3c526297d0e11a6cd6c2eee8b5a62 /mm/internal.h
parent4410cbb5c9f9371556c4e928b5dd00226b073082 (diff)
mm/numa: automatically generate node migration order
Patch series "Migrate Pages in lieu of discard", v11. We're starting to see systems with more and more kinds of memory such as Intel's implementation of persistent memory. Let's say you have a system with some DRAM and some persistent memory. Today, once DRAM fills up, reclaim will start and some of the DRAM contents will be thrown out. Allocations will, at some point, start falling over to the slower persistent memory. That has two nasty properties. First, the newer allocations can end up in the slower persistent memory. Second, reclaimed data in DRAM are just discarded even if there are gobs of space in persistent memory that could be used. This patchset implements a solution to these problems. At the end of the reclaim process in shrink_page_list() just before the last page refcount is dropped, the page is migrated to persistent memory instead of being dropped. While I've talked about a DRAM/PMEM pairing, this approach would function in any environment where memory tiers exist. This is not perfect. It "strands" pages in slower memory and never brings them back to fast DRAM. Huang Ying has follow-on work which repurposes NUMA balancing to promote hot pages back to DRAM. This is also all based on an upstream mechanism that allows persistent memory to be onlined and used as if it were volatile: With that, the DRAM and PMEM in each socket will be represented as 2 separate NUMA nodes, with the CPUs sit in the DRAM node. So the general inter-NUMA demotion mechanism introduced in the patchset can migrate the cold DRAM pages to the PMEM node. We have tested the patchset with the postgresql and pgbench. On a 2-socket server machine with DRAM and PMEM, the kernel with the patchset can improve the score of pgbench up to 22.1% compared with that of the DRAM only + disk case. This comes from the reduced disk read throughput (which reduces up to 70.8%). == Open Issues == * Memory policies and cpusets that, for instance, restrict allocations to DRAM can be demoted to PMEM whenever they opt in to this new mechanism. A cgroup-level API to opt-in or opt-out of these migrations will likely be required as a follow-on. * Could be more aggressive about where anon LRU scanning occurs since it no longer necessarily involves I/O. get_scan_count() for instance says: "If we have no swap space, do not bother scanning anon pages" This patch (of 9): Prepare for the kernel to auto-migrate pages to other memory nodes with a node migration table. This allows creating single migration target for each NUMA node to enable the kernel to do NUMA page migrations instead of simply discarding colder pages. A node with no target is a "terminal node", so reclaim acts normally there. The migration target does not fundamentally _need_ to be a single node, but this implementation starts there to limit complexity. When memory fills up on a node, memory contents can be automatically migrated to another node. The biggest problems are knowing when to migrate and to where the migration should be targeted. The most straightforward way to generate the "to where" list would be to follow the page allocator fallback lists. Those lists already tell us if memory is full where to look next. It would also be logical to move memory in that order. But, the allocator fallback lists have a fatal flaw: most nodes appear in all the lists. This would potentially lead to migration cycles (A->B, B->A, A->B, ...). Instead of using the allocator fallback lists directly, keep a separate node migration ordering. But, reuse the same data used to generate page allocator fallback in the first place: find_next_best_node(). This means that the firmware data used to populate node distances essentially dictates the ordering for now. It should also be architecture-neutral since all NUMA architectures have a working find_next_best_node(). RCU is used to allow lock-less read of node_demotion[] and prevent demotion cycles been observed. If multiple reads of node_demotion[] are performed, a single rcu_read_lock() must be held over all reads to ensure no cycles are observed. Details are as follows. === What does RCU provide? === Imagine a simple loop which walks down the demotion path looking for the last node: terminal_node = start_node; while (node_demotion[terminal_node] != NUMA_NO_NODE) { terminal_node = node_demotion[terminal_node]; } The initial values are: node_demotion[0] = 1; node_demotion[1] = NUMA_NO_NODE; and are updated to: node_demotion[0] = NUMA_NO_NODE; node_demotion[1] = 0; What guarantees that the cycle is not observed: node_demotion[0] = 1; node_demotion[1] = 0; and would loop forever? With RCU, a rcu_read_lock/unlock() can be placed around the loop. Since the write side does a synchronize_rcu(), the loop that observed the old contents is known to be complete before the synchronize_rcu() has completed. RCU, combined with disable_all_migrate_targets(), ensures that the old migration state is not visible by the time __set_migration_target_nodes() is called. === What does READ_ONCE() provide? === READ_ONCE() forbids the compiler from merging or reordering successive reads of node_demotion[]. This ensures that any updates are *eventually* observed. Consider the above loop again. The compiler could theoretically read the entirety of node_demotion[] into local storage (registers) and never go back to memory, and *permanently* observe bad values for node_demotion[]. Note: RCU does not provide any universal compiler-ordering guarantees: This code is unused for now. It will be called later in the series. Link: Link: Link: Signed-off-by: Dave Hansen <> Signed-off-by: "Huang, Ying" <> Reviewed-by: Yang Shi <> Reviewed-by: Zi Yan <> Reviewed-by: Oscar Salvador <> Cc: Michal Hocko <> Cc: Wei Xu <> Cc: David Rientjes <> Cc: Dan Williams <> Cc: David Hildenbrand <> Cc: Greg Thelen <> Cc: Keith Busch <> Cc: Yang Shi <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'mm/internal.h')
1 files changed, 5 insertions, 0 deletions
diff --git a/mm/internal.h b/mm/internal.h
index 57e28261a3b1..cf3cb933eba3 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -543,12 +543,17 @@ static inline void mminit_validate_memmodel_limits(unsigned long *start_pfn,
extern int node_reclaim(struct pglist_data *, gfp_t, unsigned int);
+extern int find_next_best_node(int node, nodemask_t *used_node_mask);
static inline int node_reclaim(struct pglist_data *pgdat, gfp_t mask,
unsigned int order)
+static inline int find_next_best_node(int node, nodemask_t *used_node_mask)
+ return NUMA_NO_NODE;
extern int hwpoison_filter(struct page *p);