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@@ -0,0 +1,280 @@
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+From 418038c22452df38cde519cc8c662bb15139764a Mon Sep 17 00:00:00 2001
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+From: Kalesh Singh <[email protected]>
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+Date: Thu, 13 Apr 2023 14:43:26 -0700
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+Subject: [PATCH 19/19] mm: Multi-gen LRU: remove wait_event_killable()
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+
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+Android 14 and later default to MGLRU [1] and field telemetry showed
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+occasional long tail latency (>100ms) in the reclaim path.
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+
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+Tracing revealed priority inversion in the reclaim path. In
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+try_to_inc_max_seq(), when high priority tasks were blocked on
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+wait_event_killable(), the preemption of the low priority task to call
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+wake_up_all() caused those high priority tasks to wait longer than
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+necessary. In general, this problem is not different from others of its
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+kind, e.g., one caused by mutex_lock(). However, it is specific to MGLRU
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+because it introduced the new wait queue lruvec->mm_state.wait.
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+
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+The purpose of this new wait queue is to avoid the thundering herd
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+problem. If many direct reclaimers rush into try_to_inc_max_seq(), only
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+one can succeed, i.e., the one to wake up the rest, and the rest who
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+failed might cause premature OOM kills if they do not wait. So far there
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+is no evidence supporting this scenario, based on how often the wait has
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+been hit. And this begs the question how useful the wait queue is in
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+practice.
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+
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+Based on Minchan's recommendation, which is in line with his commit
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+6d4675e60135 ("mm: don't be stuck to rmap lock on reclaim path") and the
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+rest of the MGLRU code which also uses trylock when possible, remove the
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+wait queue.
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+
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+[1] https://android-review.googlesource.com/q/I7ed7fbfd6ef9ce10053347528125dd98c39e50bf
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+
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+Link: https://lkml.kernel.org/r/[email protected]
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+Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
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+Signed-off-by: Kalesh Singh <[email protected]>
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+Suggested-by: Minchan Kim <[email protected]>
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+Reported-by: Wei Wang <[email protected]>
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+Acked-by: Yu Zhao <[email protected]>
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+Cc: Minchan Kim <[email protected]>
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+Cc: Jan Alexander Steffens (heftig) <[email protected]>
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+Cc: Oleksandr Natalenko <[email protected]>
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+Cc: Suleiman Souhlal <[email protected]>
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+Cc: Suren Baghdasaryan <[email protected]>
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+Signed-off-by: Andrew Morton <[email protected]>
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+---
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+ include/linux/mmzone.h | 8 +--
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+ mm/vmscan.c | 112 +++++++++++++++--------------------------
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+ 2 files changed, 42 insertions(+), 78 deletions(-)
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+
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+diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
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+index 403c7461e7a70..d62a5accf1be4 100644
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+--- a/include/linux/mmzone.h
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++++ b/include/linux/mmzone.h
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+@@ -453,18 +453,14 @@ enum {
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+ struct lru_gen_mm_state {
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+ /* set to max_seq after each iteration */
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+ unsigned long seq;
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+- /* where the current iteration continues (inclusive) */
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++ /* where the current iteration continues after */
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+ struct list_head *head;
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+- /* where the last iteration ended (exclusive) */
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++ /* where the last iteration ended before */
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+ struct list_head *tail;
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+- /* to wait for the last page table walker to finish */
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+- struct wait_queue_head wait;
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+ /* Bloom filters flip after each iteration */
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+ unsigned long *filters[NR_BLOOM_FILTERS];
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+ /* the mm stats for debugging */
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+ unsigned long stats[NR_HIST_GENS][NR_MM_STATS];
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+- /* the number of concurrent page table walkers */
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+- int nr_walkers;
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+ };
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+
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+ struct lru_gen_mm_walk {
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+diff --git a/mm/vmscan.c b/mm/vmscan.c
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+index f6ce7a1fd78a3..851758303dbf4 100644
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+--- a/mm/vmscan.c
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++++ b/mm/vmscan.c
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+@@ -3371,18 +3371,13 @@ void lru_gen_del_mm(struct mm_struct *mm)
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+ if (!lruvec)
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+ continue;
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+
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+- /* where the last iteration ended (exclusive) */
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++ /* where the current iteration continues after */
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++ if (lruvec->mm_state.head == &mm->lru_gen.list)
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++ lruvec->mm_state.head = lruvec->mm_state.head->prev;
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++
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++ /* where the last iteration ended before */
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+ if (lruvec->mm_state.tail == &mm->lru_gen.list)
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+ lruvec->mm_state.tail = lruvec->mm_state.tail->next;
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+-
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+- /* where the current iteration continues (inclusive) */
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+- if (lruvec->mm_state.head != &mm->lru_gen.list)
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+- continue;
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+-
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+- lruvec->mm_state.head = lruvec->mm_state.head->next;
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+- /* the deletion ends the current iteration */
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+- if (lruvec->mm_state.head == &mm_list->fifo)
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+- WRITE_ONCE(lruvec->mm_state.seq, lruvec->mm_state.seq + 1);
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+ }
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+
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+ list_del_init(&mm->lru_gen.list);
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+@@ -3478,68 +3473,54 @@ static bool iterate_mm_list(struct lruvec *lruvec, struct lru_gen_mm_walk *walk,
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+ struct mm_struct **iter)
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+ {
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+ bool first = false;
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+- bool last = true;
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++ bool last = false;
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+ struct mm_struct *mm = NULL;
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+ struct mem_cgroup *memcg = lruvec_memcg(lruvec);
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+ struct lru_gen_mm_list *mm_list = get_mm_list(memcg);
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+ struct lru_gen_mm_state *mm_state = &lruvec->mm_state;
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+
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+ /*
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+- * There are four interesting cases for this page table walker:
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+- * 1. It tries to start a new iteration of mm_list with a stale max_seq;
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+- * there is nothing left to do.
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+- * 2. It's the first of the current generation, and it needs to reset
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+- * the Bloom filter for the next generation.
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+- * 3. It reaches the end of mm_list, and it needs to increment
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+- * mm_state->seq; the iteration is done.
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+- * 4. It's the last of the current generation, and it needs to reset the
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+- * mm stats counters for the next generation.
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++ * mm_state->seq is incremented after each iteration of mm_list. There
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++ * are three interesting cases for this page table walker:
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++ * 1. It tries to start a new iteration with a stale max_seq: there is
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++ * nothing left to do.
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++ * 2. It started the next iteration: it needs to reset the Bloom filter
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++ * so that a fresh set of PTE tables can be recorded.
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++ * 3. It ended the current iteration: it needs to reset the mm stats
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++ * counters and tell its caller to increment max_seq.
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+ */
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+ spin_lock(&mm_list->lock);
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+
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+ VM_WARN_ON_ONCE(mm_state->seq + 1 < walk->max_seq);
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+- VM_WARN_ON_ONCE(*iter && mm_state->seq > walk->max_seq);
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+- VM_WARN_ON_ONCE(*iter && !mm_state->nr_walkers);
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+
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+- if (walk->max_seq <= mm_state->seq) {
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+- if (!*iter)
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+- last = false;
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++ if (walk->max_seq <= mm_state->seq)
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+ goto done;
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+- }
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+
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+- if (!mm_state->nr_walkers) {
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+- VM_WARN_ON_ONCE(mm_state->head && mm_state->head != &mm_list->fifo);
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++ if (!mm_state->head)
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++ mm_state->head = &mm_list->fifo;
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+
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+- mm_state->head = mm_list->fifo.next;
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++ if (mm_state->head == &mm_list->fifo)
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+ first = true;
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+- }
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+-
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+- while (!mm && mm_state->head != &mm_list->fifo) {
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+- mm = list_entry(mm_state->head, struct mm_struct, lru_gen.list);
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+
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++ do {
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+ mm_state->head = mm_state->head->next;
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++ if (mm_state->head == &mm_list->fifo) {
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++ WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
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++ last = true;
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++ break;
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++ }
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+
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+ /* force scan for those added after the last iteration */
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+- if (!mm_state->tail || mm_state->tail == &mm->lru_gen.list) {
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+- mm_state->tail = mm_state->head;
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++ if (!mm_state->tail || mm_state->tail == mm_state->head) {
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++ mm_state->tail = mm_state->head->next;
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+ walk->force_scan = true;
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+ }
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+
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++ mm = list_entry(mm_state->head, struct mm_struct, lru_gen.list);
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+ if (should_skip_mm(mm, walk))
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+ mm = NULL;
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+- }
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+-
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+- if (mm_state->head == &mm_list->fifo)
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+- WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
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++ } while (!mm);
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+ done:
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+- if (*iter && !mm)
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+- mm_state->nr_walkers--;
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+- if (!*iter && mm)
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+- mm_state->nr_walkers++;
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+-
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+- if (mm_state->nr_walkers)
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+- last = false;
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+-
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+ if (*iter || last)
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+ reset_mm_stats(lruvec, walk, last);
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+
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+@@ -3567,9 +3548,9 @@ static bool iterate_mm_list_nowalk(struct lruvec *lruvec, unsigned long max_seq)
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+
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+ VM_WARN_ON_ONCE(mm_state->seq + 1 < max_seq);
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+
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+- if (max_seq > mm_state->seq && !mm_state->nr_walkers) {
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+- VM_WARN_ON_ONCE(mm_state->head && mm_state->head != &mm_list->fifo);
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+-
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++ if (max_seq > mm_state->seq) {
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++ mm_state->head = NULL;
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++ mm_state->tail = NULL;
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+ WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
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+ reset_mm_stats(lruvec, NULL, true);
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+ success = true;
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+@@ -4172,10 +4153,6 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end,
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+
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+ walk_pmd_range(&val, addr, next, args);
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+
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+- /* a racy check to curtail the waiting time */
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+- if (wq_has_sleeper(&walk->lruvec->mm_state.wait))
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+- return 1;
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+-
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+ if (need_resched() || walk->batched >= MAX_LRU_BATCH) {
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+ end = (addr | ~PUD_MASK) + 1;
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+ goto done;
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+@@ -4208,8 +4185,14 @@ static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_
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+ walk->next_addr = FIRST_USER_ADDRESS;
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+
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+ do {
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++ DEFINE_MAX_SEQ(lruvec);
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++
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+ err = -EBUSY;
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+
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++ /* another thread might have called inc_max_seq() */
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++ if (walk->max_seq != max_seq)
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++ break;
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++
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+ /* folio_update_gen() requires stable folio_memcg() */
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+ if (!mem_cgroup_trylock_pages(memcg))
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+ break;
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+@@ -4442,25 +4425,12 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long max_seq,
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+ success = iterate_mm_list(lruvec, walk, &mm);
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+ if (mm)
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+ walk_mm(lruvec, mm, walk);
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+-
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+- cond_resched();
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+ } while (mm);
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+ done:
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+- if (!success) {
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+- if (sc->priority <= DEF_PRIORITY - 2)
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+- wait_event_killable(lruvec->mm_state.wait,
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+- max_seq < READ_ONCE(lrugen->max_seq));
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+- return false;
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+- }
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++ if (success)
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++ inc_max_seq(lruvec, can_swap, force_scan);
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+
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+- VM_WARN_ON_ONCE(max_seq != READ_ONCE(lrugen->max_seq));
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+-
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+- inc_max_seq(lruvec, can_swap, force_scan);
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+- /* either this sees any waiters or they will see updated max_seq */
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+- if (wq_has_sleeper(&lruvec->mm_state.wait))
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+- wake_up_all(&lruvec->mm_state.wait);
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+-
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+- return true;
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++ return success;
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+ }
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+
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+ /******************************************************************************
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+@@ -6105,7 +6075,6 @@ void lru_gen_init_lruvec(struct lruvec *lruvec)
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+ INIT_LIST_HEAD(&lrugen->folios[gen][type][zone]);
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+
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+ lruvec->mm_state.seq = MIN_NR_GENS;
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+- init_waitqueue_head(&lruvec->mm_state.wait);
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+ }
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+
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+ #ifdef CONFIG_MEMCG
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+@@ -6138,7 +6107,6 @@ void lru_gen_exit_memcg(struct mem_cgroup *memcg)
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+ for_each_node(nid) {
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+ struct lruvec *lruvec = get_lruvec(memcg, nid);
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+
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+- VM_WARN_ON_ONCE(lruvec->mm_state.nr_walkers);
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+ VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
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+ sizeof(lruvec->lrugen.nr_pages)));
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+
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+--
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+2.40.1
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+
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Kazuki Hashimoto