[RFC PATCH v3 7/8] mm: add kernel aging thread for workingset reporting

[Yuanchu Xie]

For reliable and timely aging on memcgs, one has to read
the page age histograms on time. A kernel thread makes it easier
by aging memcgs with valid page_age_intervals and refresh_interval
when they can be refreshed, and also reduces the latency of any
userspace consumers of the page age histogram.

The kernel aging thread is gated behind CONFIG_WORKINGSET_REPORT_AGING.
Debugging stats may be added in the future for when aging cannot
keep up with the configured refresh_interval.

Signed-off-by: Yuanchu Xie <yuanchu@xxxxxxxxxx>
---
 include/linux/workingset_report.h |  11 ++-
 mm/Kconfig                        |   6 ++
 mm/Makefile                       |   1 +
 mm/memcontrol.c                   |  11 ++-
 mm/workingset_report.c            |  14 +++-
 mm/workingset_report_aging.c      | 127 ++++++++++++++++++++++++++++++
 6 files changed, 163 insertions(+), 7 deletions(-)
 create mode 100644 mm/workingset_report_aging.c

diff --git a/include/linux/workingset_report.h b/include/linux/workingset_report.h
index e908c5678b1e..759486a3a285 100644
--- a/include/linux/workingset_report.h
+++ b/include/linux/workingset_report.h
@@ -77,9 +77,18 @@ void wsr_destroy(struct lruvec *lruvec);
  * The next refresh time is stored in refresh_time.
  */
 bool wsr_refresh_report(struct wsr_state *wsr, struct mem_cgroup *root,
-			struct pglist_data *pgdat);
+			struct pglist_data *pgdat, unsigned long *refresh_time);
 void wsr_register_node(struct node *node);
 void wsr_unregister_node(struct node *node);
+
+#ifdef CONFIG_WORKINGSET_REPORT_AGING
+void wsr_wakeup_aging_thread(void);
+#else /* CONFIG_WORKINGSET_REPORT_AGING */
+static inline void wsr_wakeup_aging_thread(void)
+{
+}
+#endif /* CONFIG_WORKINGSET_REPORT_AGING */
+
 #else
 static inline void wsr_init(struct lruvec *lruvec)
 {
diff --git a/mm/Kconfig b/mm/Kconfig
index 212f203b10b9..1e6aa1bd63f2 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1270,6 +1270,12 @@ config WORKINGSET_REPORT
 	  This option exports stats and events giving the user more insight
 	  into its memory working set.
 
+config WORKINGSET_REPORT_AGING
+	bool "Workingset report kernel aging thread"
+	depends on WORKINGSET_REPORT
+	help
+	  Performs aging on memcgs with their configured refresh intervals.
+
 source "mm/damon/Kconfig"
 
 endmenu
diff --git a/mm/Makefile b/mm/Makefile
index 57093657030d..7caae7f2d6cf 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -93,6 +93,7 @@ obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
 obj-$(CONFIG_PAGE_COUNTER) += page_counter.o
 obj-$(CONFIG_MEMCG) += memcontrol.o vmpressure.o
 obj-$(CONFIG_WORKINGSET_REPORT) += workingset_report.o
+obj-$(CONFIG_WORKINGSET_REPORT_AGING) += workingset_report_aging.o
 ifdef CONFIG_SWAP
 obj-$(CONFIG_MEMCG) += swap_cgroup.o
 endif
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2a39a4445bb7..86e15b9fc8e2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7102,6 +7102,8 @@ static ssize_t memory_ws_page_age_intervals_write(struct kernfs_open_file *of,
 	old = xchg(&wsr->page_age, page_age);
 	mutex_unlock(&wsr->page_age_lock);
 	kfree(old);
+	if (err && READ_ONCE(wsr->refresh_interval))
+		wsr_wakeup_aging_thread();
 	return nbytes;
 failed:
 	kfree(page_age);
@@ -7227,14 +7229,17 @@ static ssize_t memory_ws_refresh_interval_write(struct kernfs_open_file *of,
 {
 	unsigned int nid, msecs;
 	struct wsr_state *wsr;
+	unsigned long old_interval;
 	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
 	ssize_t ret = memory_wsr_threshold_parse(buf, nbytes, &nid, &msecs);
 
 	if (ret < 0)
 		return ret;
-
 	wsr = &mem_cgroup_lruvec(memcg, NODE_DATA(nid))->wsr;
+	old_interval = jiffies_to_msecs(READ_ONCE(wsr->refresh_interval));
 	WRITE_ONCE(wsr->refresh_interval, msecs_to_jiffies(msecs));
+	if (msecs && (!old_interval || jiffies_to_msecs(old_interval) > msecs))
+		wsr_wakeup_aging_thread();
 	return ret;
 }
 
@@ -7285,7 +7290,7 @@ static int memory_ws_page_age_show(struct seq_file *m, void *v)
 		if (!READ_ONCE(wsr->page_age))
 			continue;
 
-		wsr_refresh_report(wsr, memcg, NODE_DATA(nid));
+		wsr_refresh_report(wsr, memcg, NODE_DATA(nid), NULL);
 		mutex_lock(&wsr->page_age_lock);
 		if (!wsr->page_age)
 			goto unlock;
@@ -7325,7 +7330,7 @@ static int memory_ws_reaccess_histogram_show(struct seq_file *m, void *v)
 		if (!reaccess)
 			goto unlock;
 
-		wsr_refresh_report(wsr, memcg, NODE_DATA(nid));
+		wsr_refresh_report(wsr, memcg, NODE_DATA(nid), NULL);
 
 		seq_printf(m, "N%d\n", nid);
 		for (bin = reaccess->bins.bins;
diff --git a/mm/workingset_report.c b/mm/workingset_report.c
index 504d840bbe6a..da658967eac2 100644
--- a/mm/workingset_report.c
+++ b/mm/workingset_report.c
@@ -234,7 +234,7 @@ static void refresh_aggregate(struct wsr_page_age_histo *page_age,
 }
 
 bool wsr_refresh_report(struct wsr_state *wsr, struct mem_cgroup *root,
-			struct pglist_data *pgdat)
+			struct pglist_data *pgdat, unsigned long *refresh_time)
 {
 	struct wsr_page_age_histo *page_age = NULL;
 	unsigned long refresh_interval = READ_ONCE(wsr->refresh_interval);
@@ -253,6 +253,8 @@ bool wsr_refresh_report(struct wsr_state *wsr, struct mem_cgroup *root,
 		goto unlock;
 	refresh_scan(wsr, root, pgdat, refresh_interval);
 	refresh_aggregate(page_age, root, pgdat);
+	if (refresh_time)
+		*refresh_time = page_age->timestamp + refresh_interval;
 
 unlock:
 	mutex_unlock(&wsr->page_age_lock);
@@ -564,12 +566,16 @@ static ssize_t refresh_interval_store(struct kobject *kobj,
 	unsigned int interval;
 	int err;
 	struct wsr_state *wsr = kobj_to_wsr(kobj);
+	unsigned long old_interval;
 
 	err = kstrtouint(buf, 0, &interval);
 	if (err)
 		return err;
 
-	WRITE_ONCE(wsr->refresh_interval, msecs_to_jiffies(interval));
+	old_interval = xchg(&wsr->refresh_interval, msecs_to_jiffies(interval));
+	if (interval &&
+	    (!old_interval || jiffies_to_msecs(old_interval) > interval))
+		wsr_wakeup_aging_thread();
 
 	return len;
 }
@@ -642,6 +648,8 @@ static ssize_t page_age_intervals_store(struct kobject *kobj,
 	mutex_unlock(&wsr->page_age_lock);
 	kfree(old);
 	kfree(buf);
+	if (err && READ_ONCE(wsr->refresh_interval))
+		wsr_wakeup_aging_thread();
 	return len;
 failed:
 	kfree(page_age);
@@ -663,7 +671,7 @@ static ssize_t page_age_show(struct kobject *kobj, struct kobj_attribute *attr,
 	if (!READ_ONCE(wsr->page_age))
 		return -EINVAL;
 
-	wsr_refresh_report(wsr, NULL, kobj_to_pgdat(kobj));
+	wsr_refresh_report(wsr, NULL, kobj_to_pgdat(kobj), NULL);
 
 	mutex_lock(&wsr->page_age_lock);
 	if (!wsr->page_age) {
diff --git a/mm/workingset_report_aging.c b/mm/workingset_report_aging.c
new file mode 100644
index 000000000000..91ad5020778a
--- /dev/null
+++ b/mm/workingset_report_aging.c
@@ -0,0 +1,127 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Workingset report kernel aging thread
+ *
+ * Performs aging on behalf of memcgs with their configured refresh interval.
+ * While a userspace program can periodically read the page age breakdown
+ * per-memcg and trigger aging, the kernel performing aging is less overhead,
+ * more consistent, and more reliable for the use case where every memcg should
+ * be aged according to their refresh interval.
+ */
+#define pr_fmt(fmt) "workingset report aging: " fmt
+
+#include <linux/jiffies.h>
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/kthread.h>
+#include <linux/memcontrol.h>
+#include <linux/swap.h>
+#include <linux/wait.h>
+#include <linux/mmzone.h>
+#include <linux/workingset_report.h>
+
+static DECLARE_WAIT_QUEUE_HEAD(aging_wait);
+static bool refresh_pending;
+
+static bool do_aging_node(int nid, unsigned long *next_wake_time)
+{
+	struct mem_cgroup *memcg;
+	bool should_wait = true;
+	struct pglist_data *pgdat = NODE_DATA(nid);
+
+	memcg = mem_cgroup_iter(NULL, NULL, NULL);
+	do {
+		struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
+		struct wsr_state *wsr = &lruvec->wsr;
+		unsigned long refresh_time;
+
+		/* use returned time to decide when to wake up next */
+		if (wsr_refresh_report(wsr, memcg, pgdat, &refresh_time)) {
+			if (should_wait) {
+				should_wait = false;
+				*next_wake_time = refresh_time;
+			} else if (time_before(refresh_time, *next_wake_time)) {
+				*next_wake_time = refresh_time;
+			}
+		}
+
+		cond_resched();
+	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)));
+
+	return should_wait;
+}
+
+static int do_aging(void *unused)
+{
+	while (!kthread_should_stop()) {
+		int nid;
+		long timeout_ticks;
+		unsigned long next_wake_time;
+		bool should_wait = true;
+
+		WRITE_ONCE(refresh_pending, false);
+		for_each_node_state(nid, N_MEMORY) {
+			unsigned long node_next_wake_time;
+
+			if (do_aging_node(nid, &node_next_wake_time))
+				continue;
+			if (should_wait) {
+				should_wait = false;
+				next_wake_time = node_next_wake_time;
+			} else if (time_before(node_next_wake_time,
+					       next_wake_time)) {
+				next_wake_time = node_next_wake_time;
+			}
+		}
+
+		if (should_wait) {
+			wait_event_interruptible(aging_wait, refresh_pending);
+			continue;
+		}
+
+		/* sleep until next aging */
+		timeout_ticks = next_wake_time - jiffies;
+		if (timeout_ticks > 0 &&
+		    timeout_ticks != MAX_SCHEDULE_TIMEOUT) {
+			schedule_timeout_idle(timeout_ticks);
+			continue;
+		}
+	}
+	return 0;
+}
+
+/* Invoked when refresh_interval shortens or changes to a non-zero value. */
+void wsr_wakeup_aging_thread(void)
+{
+	WRITE_ONCE(refresh_pending, true);
+	wake_up_interruptible(&aging_wait);
+}
+
+static struct task_struct *aging_thread;
+
+static int aging_init(void)
+{
+	struct task_struct *task;
+
+	task = kthread_run(do_aging, NULL, "kagingd");
+
+	if (IS_ERR(task)) {
+		pr_err("Failed to create aging kthread\n");
+		return PTR_ERR(task);
+	}
+
+	aging_thread = task;
+	pr_info("module loaded\n");
+	return 0;
+}
+
+static void aging_exit(void)
+{
+	kthread_stop(aging_thread);
+	aging_thread = NULL;
+	pr_info("module unloaded\n");
+}
+
+module_init(aging_init);
+module_exit(aging_exit);
-- 
2.44.0.396.g6e790dbe36-goog