lru_gen_use_mm() marks an mm used on all nodes at every context switch, so each node's aging walker independently walks every mm's page tables. For an mm with memory on only a subset of nodes, the other nodes' walks find no folio belonging to that lruvec (node+memcg) - pure waste (~80% of walks on multi-socket systems). Add per-mm, per-node empty-walk tracking so get_next_mm() skips an mm on a node for K generations (K=MGLRU_EMPTY_SKIP_GENS=4) after an empty walk, then force a rescan. empty_map records the node, empty_map_seq the oldest max_seq among the set bits (via min(), so the rescan never fires late). A walk is "empty" when it finds no folio for this lruvec, regardless of MM_LEAF_TOTAL - the PMD-level Bloom filter often keeps leaf_total at 0 for a foreign mm, so gating on it would mean the skip never engages. The periodic rescan bounds the blind window to K generations, but a page that appears on a node during the skip (e.g. migration) is not aged until the rescan. A follow-up patch in this series invalidates the skip on page fault and migration. mm_struct grows by 16 bytes per process. Signed-off-by: Baoquan He --- include/linux/mm_types.h | 13 ++++++++++ mm/vmscan.c | 54 +++++++++++++++++++++++++++++++++++++--- 2 files changed, 63 insertions(+), 4 deletions(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index b5d4cd3b067b..68ec8bb2ab71 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -1410,6 +1410,17 @@ struct mm_struct { * page table walkers cleared the corresponding bits. */ unsigned long bitmap; + /* + * Cross-node empty-walk suppression: bit N set means + * node N's last aging walk of this mm found no folio + * for this lruvec (pure waste). Skip the mm on node N + * for up to K generations, then force a rescan. + * empty_map_seq is the oldest max_seq among the set + * bits (min(), conservative), so a bit is cleared when + * max_seq >= empty_map_seq + K. + */ + unsigned long empty_map; + unsigned long empty_map_seq; #ifdef CONFIG_MEMCG /* points to the memcg of "owner" above */ struct mem_cgroup *memcg; @@ -1503,6 +1514,8 @@ static inline void lru_gen_init_mm(struct mm_struct *mm) { INIT_LIST_HEAD(&mm->lru_gen.list); mm->lru_gen.bitmap = 0; + mm->lru_gen.empty_map = 0; + mm->lru_gen.empty_map_seq = ~0UL; #ifdef CONFIG_MEMCG mm->lru_gen.memcg = NULL; #endif diff --git a/mm/vmscan.c b/mm/vmscan.c index 83de2b147919..c39b392d7b7e 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2709,6 +2709,14 @@ static bool should_clear_pmd_young(void) return arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG); } +/* + * Cross-node empty walk suppression. lru_gen_use_mm() marks an mm used on all + * nodes, so aging on a node where the mm has no memory wastes a full page table + * walk. Skip such an mm for up to MGLRU_EMPTY_SKIP_GENS generations after an + * empty walk, then force-rescan to close migration/mlock/NUMA-balancing windows. + */ +#define MGLRU_EMPTY_SKIP_GENS 4 + /****************************************************************************** * shorthand helpers ******************************************************************************/ @@ -2929,9 +2937,27 @@ static struct mm_struct *get_next_mm(struct lru_gen_mm_walk *walk) mm = list_entry(mm_state->head, struct mm_struct, lru_gen.list); key = pgdat->node_id % BITS_PER_TYPE(mm->lru_gen.bitmap); + /* skip if this mm hasn't been used on this node since the last walk */ if (!walk->force_scan && !test_bit(key, &mm->lru_gen.bitmap)) return NULL; + /* + * Skip if this node's last walk of this mm was empty and fewer than K + * generations have passed; after K, clear the bit to force a rescan. + * empty_map_seq tracks the oldest marking via min(), so the rescan + * never fires later than K generations on any node. + */ + if (!walk->force_scan && test_bit(key, &mm->lru_gen.empty_map)) { + DEFINE_MAX_SEQ(walk->lruvec); + unsigned long empty_seq = READ_ONCE(mm->lru_gen.empty_map_seq); + + if (max_seq < empty_seq + MGLRU_EMPTY_SKIP_GENS) + return NULL; /* skip: < K gens since empty */ + + /* K generations passed → force rescan */ + clear_bit(key, &mm->lru_gen.empty_map); + } + clear_bit(key, &mm->lru_gen.bitmap); mmgrab(mm); @@ -4113,18 +4139,38 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long seq, do { success = iterate_mm_list(walk, &mm); if (mm) { + int nid = lruvec_pgdat(lruvec)->node_id; + int key = nid % BITS_PER_TYPE(mm->lru_gen.bitmap); bool empty = false; walk_mm(mm, walk); - /* A walk that traversed page tables but found no folio - * belonging to this lruvec (node+memcg) is pure waste. */ + + /* + * Any walk that found no folio for this lruvec is empty - + * even if the PMD-level Bloom filter kept MM_LEAF_TOTAL at 0 + * (the common case for a foreign mm). Mark it so get_next_mm() + * skips it next time; otherwise the empty-walk skip never + * engages for the walks that matter most. + */ + if (walk->mm_stats[MM_LEAF_ELIGIBLE] == 0) { + set_bit(key, &mm->lru_gen.empty_map); + /* track the oldest marking (conservative) */ + WRITE_ONCE(mm->lru_gen.empty_map_seq, + min(READ_ONCE(mm->lru_gen.empty_map_seq), + walk->seq)); + empty = true; + } else { + /* found eligible folios: clear the marking */ + clear_bit(key, &mm->lru_gen.empty_map); + } + + /* measurement stats keep the leaf_total gate */ if (walk->mm_stats[MM_LEAF_TOTAL]) { walk->mm_stats[MM_WALK_TOTAL]++; - if (walk->mm_stats[MM_LEAF_ELIGIBLE] == 0) { + if (empty) { walk->mm_stats[MM_WALK_EMPTY]++; walk->mm_stats[MM_LEAF_TOTAL_EMPTY] += walk->mm_stats[MM_LEAF_TOTAL]; - empty = true; } } trace_mm_vmscan_lru_gen_walk( -- 2.54.0