[PATCH v13 11/14] mm: multi-gen LRU: thrashing prevention

[Yu Zhao]

Add /sys/kernel/mm/lru_gen/min_ttl_ms for thrashing prevention, as
requested by many desktop users [1].

When set to value N, it prevents the working set of N milliseconds
from getting evicted. The OOM killer is triggered if this working set
cannot be kept in memory. Based on the average human detectable lag
(~100ms), N=1000 usually eliminates intolerable lags due to thrashing.
Larger values like N=3000 make lags less noticeable at the risk of
premature OOM kills.

Compared with the size-based approach [2], this time-based approach
has the following advantages:
1. It is easier to configure because it is agnostic to applications
   and memory sizes.
2. It is more reliable because it is directly wired to the OOM killer.

[1] https://lore.kernel.org/r/Ydza%2FzXKY9ATRoh6@xxxxxxxxxx/
[2] https://lore.kernel.org/r/20101028191523.GA14972@xxxxxxxxxx/

Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
Acked-by: Brian Geffon <bgeffon@xxxxxxxxxx>
Acked-by: Jan Alexander Steffens (heftig) <heftig@xxxxxxxxxxxxx>
Acked-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
Acked-by: Steven Barrett <steven@xxxxxxxxxxxx>
Acked-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
Tested-by: Daniel Byrne <djbyrne@xxxxxxx>
Tested-by: Donald Carr <d@xxxxxxxxxxxxxxx>
Tested-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>
Tested-by: Konstantin Kharlamov <Hi-Angel@xxxxxxxxx>
Tested-by: Shuang Zhai <szhai2@xxxxxxxxxxxxxxxx>
Tested-by: Sofia Trinh <sofia.trinh@edi.works>
Tested-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>
---
 include/linux/mmzone.h |  2 ++
 mm/vmscan.c            | 71 +++++++++++++++++++++++++++++++++++++++---
 2 files changed, 69 insertions(+), 4 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 840b7ca8b91f..472bd5335517 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -419,6 +419,8 @@ struct lru_gen_struct {
 	unsigned long max_seq;
 	/* the eviction increments the oldest generation numbers */
 	unsigned long min_seq[ANON_AND_FILE];
+	/* the birth time of each generation in jiffies */
+	unsigned long timestamps[MAX_NR_GENS];
 	/* the multi-gen LRU lists, lazily sorted on eviction */
 	struct list_head lists[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES];
 	/* the multi-gen LRU sizes, eventually consistent */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4c8b475429ed..1f2892a0dc41 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4233,6 +4233,7 @@ static void inc_max_seq(struct lruvec *lruvec, bool can_swap)
 	for (type = 0; type < ANON_AND_FILE; type++)
 		reset_ctrl_pos(lruvec, type, false);
 
+	WRITE_ONCE(lrugen->timestamps[next], jiffies);
 	/* make sure preceding modifications appear */
 	smp_store_release(&lrugen->max_seq, lrugen->max_seq + 1);
 
@@ -4359,7 +4360,7 @@ static unsigned long get_nr_evictable(struct lruvec *lruvec, unsigned long max_s
 	return total;
 }
 
-static void age_lruvec(struct lruvec *lruvec, struct scan_control *sc)
+static bool age_lruvec(struct lruvec *lruvec, struct scan_control *sc, unsigned long min_ttl)
 {
 	bool need_aging;
 	unsigned long nr_to_scan;
@@ -4373,21 +4374,40 @@ static void age_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 	mem_cgroup_calculate_protection(NULL, memcg);
 
 	if (mem_cgroup_below_min(memcg))
-		return;
+		return false;
 
 	nr_to_scan = get_nr_evictable(lruvec, max_seq, min_seq, swappiness, &need_aging);
 	if (!nr_to_scan)
-		return;
+		return false;
 
 	nr_to_scan >>= mem_cgroup_online(memcg) ? sc->priority : 0;
 
+	if (min_ttl) {
+		int gen = lru_gen_from_seq(min_seq[LRU_GEN_FILE]);
+		unsigned long birth = READ_ONCE(lruvec->lrugen.timestamps[gen]);
+
+		if (time_is_after_jiffies(birth + min_ttl))
+			return false;
+
+		/* the size is likely too small to be helpful */
+		if (!nr_to_scan && sc->priority != DEF_PRIORITY)
+			return false;
+	}
+
 	if (nr_to_scan && need_aging)
 		try_to_inc_max_seq(lruvec, max_seq, sc, swappiness);
+
+	return true;
 }
 
+/* to protect the working set of the last N jiffies */
+static unsigned long lru_gen_min_ttl __read_mostly;
+
 static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
 {
 	struct mem_cgroup *memcg;
+	bool success = false;
+	unsigned long min_ttl = READ_ONCE(lru_gen_min_ttl);
 
 	VM_WARN_ON_ONCE(!current_is_kswapd());
 
@@ -4413,12 +4433,28 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
 	do {
 		struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
-		age_lruvec(lruvec, sc);
+		if (age_lruvec(lruvec, sc, min_ttl))
+			success = true;
 
 		cond_resched();
 	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)));
 
 	clear_mm_walk();
+
+	/*
+	 * The main goal is to OOM kill if every generation from all memcgs is
+	 * younger than min_ttl. However, another theoretical possibility is all
+	 * memcgs are either below min or empty.
+	 */
+	if (!success && !sc->order && mutex_trylock(&oom_lock)) {
+		struct oom_control oc = {
+			.gfp_mask = sc->gfp_mask,
+		};
+
+		out_of_memory(&oc);
+
+		mutex_unlock(&oom_lock);
+	}
 }
 
 /*
@@ -5135,6 +5171,28 @@ static void lru_gen_change_state(bool enabled)
  *                          sysfs interface
  ******************************************************************************/
 
+static ssize_t show_min_ttl(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%u\n", jiffies_to_msecs(READ_ONCE(lru_gen_min_ttl)));
+}
+
+static ssize_t store_min_ttl(struct kobject *kobj, struct kobj_attribute *attr,
+			     const char *buf, size_t len)
+{
+	unsigned int msecs;
+
+	if (kstrtouint(buf, 0, &msecs))
+		return -EINVAL;
+
+	WRITE_ONCE(lru_gen_min_ttl, msecs_to_jiffies(msecs));
+
+	return len;
+}
+
+static struct kobj_attribute lru_gen_min_ttl_attr = __ATTR(
+	min_ttl_ms, 0644, show_min_ttl, store_min_ttl
+);
+
 static ssize_t show_enabled(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
 {
 	unsigned int caps = 0;
@@ -5183,6 +5241,7 @@ static struct kobj_attribute lru_gen_enabled_attr = __ATTR(
 );
 
 static struct attribute *lru_gen_attrs[] = {
+	&lru_gen_min_ttl_attr.attr,
 	&lru_gen_enabled_attr.attr,
 	NULL
 };
@@ -5198,12 +5257,16 @@ static struct attribute_group lru_gen_attr_group = {
 
 void lru_gen_init_lruvec(struct lruvec *lruvec)
 {
+	int i;
 	int gen, type, zone;
 	struct lru_gen_struct *lrugen = &lruvec->lrugen;
 
 	lrugen->max_seq = MIN_NR_GENS + 1;
 	lrugen->enabled = lru_gen_enabled();
 
+	for (i = 0; i <= MIN_NR_GENS + 1; i++)
+		lrugen->timestamps[i] = jiffies;
+
 	for_each_gen_type_zone(gen, type, zone)
 		INIT_LIST_HEAD(&lrugen->lists[gen][type][zone]);
 
-- 
2.37.0.rc0.161.g10f37bed90-goog