[PATCH v7 10/12] mm: multigenerational 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
can't 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, e.g., [2], this time-based
approach has the following advantages:
1) It's easier to configure because it's agnostic to applications and
   memory sizes.
2) It's more reliable because it's directly wired to the OOM killer.

[1] https://lore.kernel.org/lkml/Ydza%2FzXKY9ATRoh6@google.com/
[2] https://lore.kernel.org/lkml/20211130201652.2218636d@mail.inbox.lv/

Signed-off-by: Yu Zhao <yuzhao at google.com>
Acked-by: Brian Geffon <bgeffon at google.com>
Acked-by: Jan Alexander Steffens (heftig) <heftig at archlinux.org>
Acked-by: Oleksandr Natalenko <oleksandr at natalenko.name>
Acked-by: Steven Barrett <steven at liquorix.net>
Acked-by: Suleiman Souhlal <suleiman at google.com>
Tested-by: Daniel Byrne <djbyrne at mtu.edu>
Tested-by: Donald Carr <d at chaos-reins.com>
Tested-by: Holger Hoffstätte <holger at applied-asynchrony.com>
Tested-by: Konstantin Kharlamov <Hi-Angel at yandex.ru>
Tested-by: Shuang Zhai <szhai2 at cs.rochester.edu>
Tested-by: Sofia Trinh <sofia.trinh at edi.works>
---
 mm/vmscan.c | 64 +++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 60 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 700c35f2a030..4d37d63668b5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4335,7 +4335,8 @@ static long get_nr_evictable(struct lruvec *lruvec, unsigned long max_seq,
 	return total > 0 ? total : 0;
 }
 
-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;
 	long nr_to_scan;
@@ -4344,14 +4345,22 @@ static void age_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 	DEFINE_MAX_SEQ(lruvec);
 	DEFINE_MIN_SEQ(lruvec);
 
+	if (min_ttl) {
+		int gen = lru_gen_from_seq(min_seq[TYPE_FILE]);
+		unsigned long birth = READ_ONCE(lruvec->lrugen.timestamps[gen]);
+
+		if (time_is_after_jiffies(birth + min_ttl))
+			return false;
+	}
+
 	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 >>= sc->priority;
 
@@ -4360,11 +4369,18 @@ static void age_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 
 	if (nr_to_scan && need_aging && (!mem_cgroup_below_low(memcg) || sc->memcg_low_reclaim))
 		try_to_inc_max_seq(lruvec, max_seq, sc, swappiness, false);
+
+	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_BUG_ON(!current_is_kswapd());
 
@@ -4390,11 +4406,28 @@ 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)));
 
+	/*
+	 * 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 && mutex_trylock(&oom_lock)) {
+		struct oom_control oc = {
+			.gfp_mask = sc->gfp_mask,
+			.order = sc->order,
+		};
+
+		out_of_memory(&oc);
+
+		mutex_unlock(&oom_lock);
+	}
+
 	current->reclaim_state->mm_walk = NULL;
 }
 
@@ -5107,6 +5140,28 @@ static void lru_gen_change_state(bool enable)
  *                          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_enable(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
 {
 	unsigned int caps = 0;
@@ -5155,6 +5210,7 @@ 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
 };
-- 
2.35.0.263.gb82422642f-goog