--- /dev/null
+From 087ed25eaf5a78a678508e893f80addab9b1c103 Mon Sep 17 00:00:00 2001
+From: Kalesh Singh <kaleshsingh@google.com>
+Date: Thu, 13 Apr 2023 14:43:26 -0700
+Subject: [PATCH] mm: Multi-gen LRU: remove wait_event_killable()
+
+Android 14 and later default to MGLRU [1] and field telemetry showed
+occasional long tail latency (>100ms) in the reclaim path.
+
+Tracing revealed priority inversion in the reclaim path. In
+try_to_inc_max_seq(), when high priority tasks were blocked on
+wait_event_killable(), the preemption of the low priority task to call
+wake_up_all() caused those high priority tasks to wait longer than
+necessary. In general, this problem is not different from others of its
+kind, e.g., one caused by mutex_lock(). However, it is specific to MGLRU
+because it introduced the new wait queue lruvec->mm_state.wait.
+
+The purpose of this new wait queue is to avoid the thundering herd
+problem. If many direct reclaimers rush into try_to_inc_max_seq(), only
+one can succeed, i.e., the one to wake up the rest, and the rest who
+failed might cause premature OOM kills if they do not wait. So far there
+is no evidence supporting this scenario, based on how often the wait has
+been hit. And this begs the question how useful the wait queue is in
+practice.
+
+Based on Minchan's recommendation, which is in line with his commit
+6d4675e60135 ("mm: don't be stuck to rmap lock on reclaim path") and the
+rest of the MGLRU code which also uses trylock when possible, remove the
+wait queue.
+
+[1] https://android-review.googlesource.com/q/I7ed7fbfd6ef9ce10053347528125dd98c39e50bf
+
+Link: https://lkml.kernel.org/r/20230413214326.2147568-1-kaleshsingh@google.com
+Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
+Change-Id: I911f3968fd1adb25171279cc5b6f48ccb7efc8de
+Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
+Suggested-by: Minchan Kim <minchan@kernel.org>
+Reported-by: Wei Wang <wvw@google.com>
+Acked-by: Yu Zhao <yuzhao@google.com>
+Cc: Minchan Kim <minchan@kernel.org>
+Cc: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
+Cc: Oleksandr Natalenko <oleksandr@natalenko.name>
+Cc: Suleiman Souhlal <suleiman@google.com>
+Cc: Suren Baghdasaryan <surenb@google.com>
+Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
+---
+ include/linux/mmzone.h | 8 +--
+ mm/vmscan.c | 111 +++++++++++++++--------------------------
+ 2 files changed, 42 insertions(+), 77 deletions(-)
+
+--- a/include/linux/mmzone.h
++++ b/include/linux/mmzone.h
+@@ -452,18 +452,14 @@ enum {
+ struct lru_gen_mm_state {
+ /* set to max_seq after each iteration */
+ unsigned long seq;
+- /* where the current iteration continues (inclusive) */
++ /* where the current iteration continues after */
+ struct list_head *head;
+- /* where the last iteration ended (exclusive) */
++ /* where the last iteration ended before */
+ struct list_head *tail;
+- /* to wait for the last page table walker to finish */
+- struct wait_queue_head wait;
+ /* Bloom filters flip after each iteration */
+ unsigned long *filters[NR_BLOOM_FILTERS];
+ /* the mm stats for debugging */
+ unsigned long stats[NR_HIST_GENS][NR_MM_STATS];
+- /* the number of concurrent page table walkers */
+- int nr_walkers;
+ };
+
+ struct lru_gen_mm_walk {
+--- a/mm/vmscan.c
++++ b/mm/vmscan.c
+@@ -2999,18 +2999,13 @@ void lru_gen_del_mm(struct mm_struct *mm
+ if (!lruvec)
+ continue;
+
+- /* where the last iteration ended (exclusive) */
++ /* where the current iteration continues after */
++ if (lruvec->mm_state.head == &mm->lru_gen.list)
++ lruvec->mm_state.head = lruvec->mm_state.head->prev;
++
++ /* where the last iteration ended before */
+ if (lruvec->mm_state.tail == &mm->lru_gen.list)
+ lruvec->mm_state.tail = lruvec->mm_state.tail->next;
+-
+- /* where the current iteration continues (inclusive) */
+- if (lruvec->mm_state.head != &mm->lru_gen.list)
+- continue;
+-
+- lruvec->mm_state.head = lruvec->mm_state.head->next;
+- /* the deletion ends the current iteration */
+- if (lruvec->mm_state.head == &mm_list->fifo)
+- WRITE_ONCE(lruvec->mm_state.seq, lruvec->mm_state.seq + 1);
+ }
+
+ list_del_init(&mm->lru_gen.list);
+@@ -3194,68 +3189,54 @@ static bool iterate_mm_list(struct lruve
+ struct mm_struct **iter)
+ {
+ bool first = false;
+- bool last = true;
++ bool last = false;
+ struct mm_struct *mm = NULL;
+ struct mem_cgroup *memcg = lruvec_memcg(lruvec);
+ struct lru_gen_mm_list *mm_list = get_mm_list(memcg);
+ struct lru_gen_mm_state *mm_state = &lruvec->mm_state;
+
+ /*
+- * There are four interesting cases for this page table walker:
+- * 1. It tries to start a new iteration of mm_list with a stale max_seq;
+- * there is nothing left to do.
+- * 2. It's the first of the current generation, and it needs to reset
+- * the Bloom filter for the next generation.
+- * 3. It reaches the end of mm_list, and it needs to increment
+- * mm_state->seq; the iteration is done.
+- * 4. It's the last of the current generation, and it needs to reset the
+- * mm stats counters for the next generation.
++ * mm_state->seq is incremented after each iteration of mm_list. There
++ * are three interesting cases for this page table walker:
++ * 1. It tries to start a new iteration with a stale max_seq: there is
++ * nothing left to do.
++ * 2. It started the next iteration: it needs to reset the Bloom filter
++ * so that a fresh set of PTE tables can be recorded.
++ * 3. It ended the current iteration: it needs to reset the mm stats
++ * counters and tell its caller to increment max_seq.
+ */
+ spin_lock(&mm_list->lock);
+
+ VM_WARN_ON_ONCE(mm_state->seq + 1 < walk->max_seq);
+- VM_WARN_ON_ONCE(*iter && mm_state->seq > walk->max_seq);
+- VM_WARN_ON_ONCE(*iter && !mm_state->nr_walkers);
+
+- if (walk->max_seq <= mm_state->seq) {
+- if (!*iter)
+- last = false;
++ if (walk->max_seq <= mm_state->seq)
+ goto done;
+- }
+
+- if (!mm_state->nr_walkers) {
+- VM_WARN_ON_ONCE(mm_state->head && mm_state->head != &mm_list->fifo);
++ if (!mm_state->head)
++ mm_state->head = &mm_list->fifo;
+
+- mm_state->head = mm_list->fifo.next;
++ if (mm_state->head == &mm_list->fifo)
+ first = true;
+- }
+-
+- while (!mm && mm_state->head != &mm_list->fifo) {
+- mm = list_entry(mm_state->head, struct mm_struct, lru_gen.list);
+
++ do {
+ mm_state->head = mm_state->head->next;
++ if (mm_state->head == &mm_list->fifo) {
++ WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
++ last = true;
++ break;
++ }
+
+ /* force scan for those added after the last iteration */
+- if (!mm_state->tail || mm_state->tail == &mm->lru_gen.list) {
+- mm_state->tail = mm_state->head;
++ if (!mm_state->tail || mm_state->tail == mm_state->head) {
++ mm_state->tail = mm_state->head->next;
+ walk->force_scan = true;
+ }
+
++ mm = list_entry(mm_state->head, struct mm_struct, lru_gen.list);
+ if (should_skip_mm(mm, walk))
+ mm = NULL;
+- }
+-
+- if (mm_state->head == &mm_list->fifo)
+- WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
++ } while (!mm);
+ done:
+- if (*iter && !mm)
+- mm_state->nr_walkers--;
+- if (!*iter && mm)
+- mm_state->nr_walkers++;
+-
+- if (mm_state->nr_walkers)
+- last = false;
+-
+ if (*iter || last)
+ reset_mm_stats(lruvec, walk, last);
+
+@@ -3283,9 +3264,9 @@ static bool iterate_mm_list_nowalk(struc
+
+ VM_WARN_ON_ONCE(mm_state->seq + 1 < max_seq);
+
+- if (max_seq > mm_state->seq && !mm_state->nr_walkers) {
+- VM_WARN_ON_ONCE(mm_state->head && mm_state->head != &mm_list->fifo);
+-
++ if (max_seq > mm_state->seq) {
++ mm_state->head = NULL;
++ mm_state->tail = NULL;
+ WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
+ reset_mm_stats(lruvec, NULL, true);
+ success = true;
+@@ -3894,10 +3875,6 @@ restart:
+
+ walk_pmd_range(&val, addr, next, args);
+
+- /* a racy check to curtail the waiting time */
+- if (wq_has_sleeper(&walk->lruvec->mm_state.wait))
+- return 1;
+-
+ if (need_resched() || walk->batched >= MAX_LRU_BATCH) {
+ end = (addr | ~PUD_MASK) + 1;
+ goto done;
+@@ -3930,8 +3907,14 @@ static void walk_mm(struct lruvec *lruve
+ walk->next_addr = FIRST_USER_ADDRESS;
+
+ do {
++ DEFINE_MAX_SEQ(lruvec);
++
+ err = -EBUSY;
+
++ /* another thread might have called inc_max_seq() */
++ if (walk->max_seq != max_seq)
++ break;
++
+ /* page_update_gen() requires stable page_memcg() */
+ if (!mem_cgroup_trylock_pages(memcg))
+ break;
+@@ -4164,25 +4147,12 @@ static bool try_to_inc_max_seq(struct lr
+ success = iterate_mm_list(lruvec, walk, &mm);
+ if (mm)
+ walk_mm(lruvec, mm, walk);
+-
+- cond_resched();
+ } while (mm);
+ done:
+- if (!success) {
+- if (sc->priority <= DEF_PRIORITY - 2)
+- wait_event_killable(lruvec->mm_state.wait,
+- max_seq < READ_ONCE(lrugen->max_seq));
+- return false;
+- }
++ if (success)
++ inc_max_seq(lruvec, can_swap, force_scan);
+
+- VM_WARN_ON_ONCE(max_seq != READ_ONCE(lrugen->max_seq));
+-
+- inc_max_seq(lruvec, can_swap, force_scan);
+- /* either this sees any waiters or they will see updated max_seq */
+- if (wq_has_sleeper(&lruvec->mm_state.wait))
+- wake_up_all(&lruvec->mm_state.wait);
+-
+- return true;
++ return success;
+ }
+
+ static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc)
+@@ -5746,7 +5716,6 @@ void lru_gen_init_lruvec(struct lruvec *
+ INIT_LIST_HEAD(&lrugen->pages[gen][type][zone]);
+
+ lruvec->mm_state.seq = MIN_NR_GENS;
+- init_waitqueue_head(&lruvec->mm_state.wait);
+ }
+
+ #ifdef CONFIG_MEMCG
--- /dev/null
+From 418038c22452df38cde519cc8c662bb15139764a Mon Sep 17 00:00:00 2001
+From: Kalesh Singh <kaleshsingh@google.com>
+Date: Thu, 13 Apr 2023 14:43:26 -0700
+Subject: [PATCH 19/19] mm: Multi-gen LRU: remove wait_event_killable()
+
+Android 14 and later default to MGLRU [1] and field telemetry showed
+occasional long tail latency (>100ms) in the reclaim path.
+
+Tracing revealed priority inversion in the reclaim path. In
+try_to_inc_max_seq(), when high priority tasks were blocked on
+wait_event_killable(), the preemption of the low priority task to call
+wake_up_all() caused those high priority tasks to wait longer than
+necessary. In general, this problem is not different from others of its
+kind, e.g., one caused by mutex_lock(). However, it is specific to MGLRU
+because it introduced the new wait queue lruvec->mm_state.wait.
+
+The purpose of this new wait queue is to avoid the thundering herd
+problem. If many direct reclaimers rush into try_to_inc_max_seq(), only
+one can succeed, i.e., the one to wake up the rest, and the rest who
+failed might cause premature OOM kills if they do not wait. So far there
+is no evidence supporting this scenario, based on how often the wait has
+been hit. And this begs the question how useful the wait queue is in
+practice.
+
+Based on Minchan's recommendation, which is in line with his commit
+6d4675e60135 ("mm: don't be stuck to rmap lock on reclaim path") and the
+rest of the MGLRU code which also uses trylock when possible, remove the
+wait queue.
+
+[1] https://android-review.googlesource.com/q/I7ed7fbfd6ef9ce10053347528125dd98c39e50bf
+
+Link: https://lkml.kernel.org/r/20230413214326.2147568-1-kaleshsingh@google.com
+Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
+Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
+Suggested-by: Minchan Kim <minchan@kernel.org>
+Reported-by: Wei Wang <wvw@google.com>
+Acked-by: Yu Zhao <yuzhao@google.com>
+Cc: Minchan Kim <minchan@kernel.org>
+Cc: Jan Alexander Steffens (heftig) <heftig@archlinux.org>
+Cc: Oleksandr Natalenko <oleksandr@natalenko.name>
+Cc: Suleiman Souhlal <suleiman@google.com>
+Cc: Suren Baghdasaryan <surenb@google.com>
+Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
+---
+ include/linux/mmzone.h | 8 +--
+ mm/vmscan.c | 112 +++++++++++++++--------------------------
+ 2 files changed, 42 insertions(+), 78 deletions(-)
+
+diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
+index 403c7461e7a70..d62a5accf1be4 100644
+--- a/include/linux/mmzone.h
++++ b/include/linux/mmzone.h
+@@ -453,18 +453,14 @@ enum {
+ struct lru_gen_mm_state {
+ /* set to max_seq after each iteration */
+ unsigned long seq;
+- /* where the current iteration continues (inclusive) */
++ /* where the current iteration continues after */
+ struct list_head *head;
+- /* where the last iteration ended (exclusive) */
++ /* where the last iteration ended before */
+ struct list_head *tail;
+- /* to wait for the last page table walker to finish */
+- struct wait_queue_head wait;
+ /* Bloom filters flip after each iteration */
+ unsigned long *filters[NR_BLOOM_FILTERS];
+ /* the mm stats for debugging */
+ unsigned long stats[NR_HIST_GENS][NR_MM_STATS];
+- /* the number of concurrent page table walkers */
+- int nr_walkers;
+ };
+
+ struct lru_gen_mm_walk {
+diff --git a/mm/vmscan.c b/mm/vmscan.c
+index f6ce7a1fd78a3..851758303dbf4 100644
+--- a/mm/vmscan.c
++++ b/mm/vmscan.c
+@@ -3371,18 +3371,13 @@ void lru_gen_del_mm(struct mm_struct *mm)
+ if (!lruvec)
+ continue;
+
+- /* where the last iteration ended (exclusive) */
++ /* where the current iteration continues after */
++ if (lruvec->mm_state.head == &mm->lru_gen.list)
++ lruvec->mm_state.head = lruvec->mm_state.head->prev;
++
++ /* where the last iteration ended before */
+ if (lruvec->mm_state.tail == &mm->lru_gen.list)
+ lruvec->mm_state.tail = lruvec->mm_state.tail->next;
+-
+- /* where the current iteration continues (inclusive) */
+- if (lruvec->mm_state.head != &mm->lru_gen.list)
+- continue;
+-
+- lruvec->mm_state.head = lruvec->mm_state.head->next;
+- /* the deletion ends the current iteration */
+- if (lruvec->mm_state.head == &mm_list->fifo)
+- WRITE_ONCE(lruvec->mm_state.seq, lruvec->mm_state.seq + 1);
+ }
+
+ list_del_init(&mm->lru_gen.list);
+@@ -3478,68 +3473,54 @@ static bool iterate_mm_list(struct lruvec *lruvec, struct lru_gen_mm_walk *walk,
+ struct mm_struct **iter)
+ {
+ bool first = false;
+- bool last = true;
++ bool last = false;
+ struct mm_struct *mm = NULL;
+ struct mem_cgroup *memcg = lruvec_memcg(lruvec);
+ struct lru_gen_mm_list *mm_list = get_mm_list(memcg);
+ struct lru_gen_mm_state *mm_state = &lruvec->mm_state;
+
+ /*
+- * There are four interesting cases for this page table walker:
+- * 1. It tries to start a new iteration of mm_list with a stale max_seq;
+- * there is nothing left to do.
+- * 2. It's the first of the current generation, and it needs to reset
+- * the Bloom filter for the next generation.
+- * 3. It reaches the end of mm_list, and it needs to increment
+- * mm_state->seq; the iteration is done.
+- * 4. It's the last of the current generation, and it needs to reset the
+- * mm stats counters for the next generation.
++ * mm_state->seq is incremented after each iteration of mm_list. There
++ * are three interesting cases for this page table walker:
++ * 1. It tries to start a new iteration with a stale max_seq: there is
++ * nothing left to do.
++ * 2. It started the next iteration: it needs to reset the Bloom filter
++ * so that a fresh set of PTE tables can be recorded.
++ * 3. It ended the current iteration: it needs to reset the mm stats
++ * counters and tell its caller to increment max_seq.
+ */
+ spin_lock(&mm_list->lock);
+
+ VM_WARN_ON_ONCE(mm_state->seq + 1 < walk->max_seq);
+- VM_WARN_ON_ONCE(*iter && mm_state->seq > walk->max_seq);
+- VM_WARN_ON_ONCE(*iter && !mm_state->nr_walkers);
+
+- if (walk->max_seq <= mm_state->seq) {
+- if (!*iter)
+- last = false;
++ if (walk->max_seq <= mm_state->seq)
+ goto done;
+- }
+
+- if (!mm_state->nr_walkers) {
+- VM_WARN_ON_ONCE(mm_state->head && mm_state->head != &mm_list->fifo);
++ if (!mm_state->head)
++ mm_state->head = &mm_list->fifo;
+
+- mm_state->head = mm_list->fifo.next;
++ if (mm_state->head == &mm_list->fifo)
+ first = true;
+- }
+-
+- while (!mm && mm_state->head != &mm_list->fifo) {
+- mm = list_entry(mm_state->head, struct mm_struct, lru_gen.list);
+
++ do {
+ mm_state->head = mm_state->head->next;
++ if (mm_state->head == &mm_list->fifo) {
++ WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
++ last = true;
++ break;
++ }
+
+ /* force scan for those added after the last iteration */
+- if (!mm_state->tail || mm_state->tail == &mm->lru_gen.list) {
+- mm_state->tail = mm_state->head;
++ if (!mm_state->tail || mm_state->tail == mm_state->head) {
++ mm_state->tail = mm_state->head->next;
+ walk->force_scan = true;
+ }
+
++ mm = list_entry(mm_state->head, struct mm_struct, lru_gen.list);
+ if (should_skip_mm(mm, walk))
+ mm = NULL;
+- }
+-
+- if (mm_state->head == &mm_list->fifo)
+- WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
++ } while (!mm);
+ done:
+- if (*iter && !mm)
+- mm_state->nr_walkers--;
+- if (!*iter && mm)
+- mm_state->nr_walkers++;
+-
+- if (mm_state->nr_walkers)
+- last = false;
+-
+ if (*iter || last)
+ reset_mm_stats(lruvec, walk, last);
+
+@@ -3567,9 +3548,9 @@ static bool iterate_mm_list_nowalk(struct lruvec *lruvec, unsigned long max_seq)
+
+ VM_WARN_ON_ONCE(mm_state->seq + 1 < max_seq);
+
+- if (max_seq > mm_state->seq && !mm_state->nr_walkers) {
+- VM_WARN_ON_ONCE(mm_state->head && mm_state->head != &mm_list->fifo);
+-
++ if (max_seq > mm_state->seq) {
++ mm_state->head = NULL;
++ mm_state->tail = NULL;
+ WRITE_ONCE(mm_state->seq, mm_state->seq + 1);
+ reset_mm_stats(lruvec, NULL, true);
+ success = true;
+@@ -4172,10 +4153,6 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end,
+
+ walk_pmd_range(&val, addr, next, args);
+
+- /* a racy check to curtail the waiting time */
+- if (wq_has_sleeper(&walk->lruvec->mm_state.wait))
+- return 1;
+-
+ if (need_resched() || walk->batched >= MAX_LRU_BATCH) {
+ end = (addr | ~PUD_MASK) + 1;
+ goto done;
+@@ -4208,8 +4185,14 @@ static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_
+ walk->next_addr = FIRST_USER_ADDRESS;
+
+ do {
++ DEFINE_MAX_SEQ(lruvec);
++
+ err = -EBUSY;
+
++ /* another thread might have called inc_max_seq() */
++ if (walk->max_seq != max_seq)
++ break;
++
+ /* folio_update_gen() requires stable folio_memcg() */
+ if (!mem_cgroup_trylock_pages(memcg))
+ break;
+@@ -4442,25 +4425,12 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long max_seq,
+ success = iterate_mm_list(lruvec, walk, &mm);
+ if (mm)
+ walk_mm(lruvec, mm, walk);
+-
+- cond_resched();
+ } while (mm);
+ done:
+- if (!success) {
+- if (sc->priority <= DEF_PRIORITY - 2)
+- wait_event_killable(lruvec->mm_state.wait,
+- max_seq < READ_ONCE(lrugen->max_seq));
+- return false;
+- }
++ if (success)
++ inc_max_seq(lruvec, can_swap, force_scan);
+
+- VM_WARN_ON_ONCE(max_seq != READ_ONCE(lrugen->max_seq));
+-
+- inc_max_seq(lruvec, can_swap, force_scan);
+- /* either this sees any waiters or they will see updated max_seq */
+- if (wq_has_sleeper(&lruvec->mm_state.wait))
+- wake_up_all(&lruvec->mm_state.wait);
+-
+- return true;
++ return success;
+ }
+
+ /******************************************************************************
+@@ -6105,7 +6075,6 @@ void lru_gen_init_lruvec(struct lruvec *lruvec)
+ INIT_LIST_HEAD(&lrugen->folios[gen][type][zone]);
+
+ lruvec->mm_state.seq = MIN_NR_GENS;
+- init_waitqueue_head(&lruvec->mm_state.wait);
+ }
+
+ #ifdef CONFIG_MEMCG
+@@ -6138,7 +6107,6 @@ void lru_gen_exit_memcg(struct mem_cgroup *memcg)
+ for_each_node(nid) {
+ struct lruvec *lruvec = get_lruvec(memcg, nid);
+
+- VM_WARN_ON_ONCE(lruvec->mm_state.nr_walkers);
+ VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
+ sizeof(lruvec->lrugen.nr_pages)));
+
+--
+2.40.1
+
projects / openwrt / openwrt.git / commitdiff