[PATCH 5/5] dm-bufio: introduce a global cache replacement

[Mikulas Patocka <mpatocka@redhat.com>]

[-- Attachment #1: dm-bufio-global-cleanup.patch --]
[-- Type: text/plain, Size: 5648 bytes --]

This patch introduces a global cache replacement (instead of per-client
cleanup).

If one bufio client uses the cache heavily and another client is not using
it, we want to let the first client use most of the cache. The old
algorithm would partition the cache equally betwen the clients and that is
inoptimal.

For cache replacement, we use the clock algorithm because it doesn't
require taking any lock when the buffer is accessed.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-bufio.c |  103 ++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 96 insertions(+), 7 deletions(-)

Index: linux-2.6/drivers/md/dm-bufio.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-bufio.c	2019-09-11 20:32:15.000000000 +0200
+++ linux-2.6/drivers/md/dm-bufio.c	2019-09-12 10:23:57.000000000 +0200
@@ -34,6 +34,7 @@
 #define DM_BUFIO_MEMORY_PERCENT		2
 #define DM_BUFIO_VMALLOC_PERCENT	25
 #define DM_BUFIO_WRITEBACK_RATIO	3
+#define DM_BUFIO_LOW_WATERMARK		16
 
 /*
  * Check buffer ages in this interval (seconds)
@@ -140,6 +141,7 @@ struct dm_buffer {
 	unsigned char list_mode;		/* LIST_* */
 	blk_status_t read_error;
 	blk_status_t write_error;
+	unsigned accessed;
 	unsigned hold_count;
 	unsigned long state;
 	unsigned long last_accessed;
@@ -198,6 +200,8 @@ static DEFINE_SPINLOCK(global_spinlock);
 
 static LIST_HEAD(global_queue);
 
+static unsigned long global_num = 0;
+
 /*
  * Buffers are freed after this timeout
  */
@@ -227,6 +231,12 @@ static LIST_HEAD(dm_bufio_all_clients);
  */
 static DEFINE_MUTEX(dm_bufio_clients_lock);
 
+
+static struct workqueue_struct *dm_bufio_wq;
+static struct delayed_work dm_bufio_cleanup_old_work;
+static struct work_struct dm_bufio_replacement_work;
+
+
 #ifdef CONFIG_DM_DEBUG_BLOCK_STACK_TRACING
 static void buffer_record_stack(struct dm_buffer *b)
 {
@@ -308,10 +318,16 @@ static void adjust_total_allocated(struc
 	if (dm_bufio_current_allocated > dm_bufio_peak_allocated)
 		dm_bufio_peak_allocated = dm_bufio_current_allocated;
 
+	b->accessed = 1;
+
 	if (!unlink) {
 		list_add(&b->global_list, &global_queue);
+		global_num++;
+		if (dm_bufio_current_allocated > dm_bufio_cache_size)
+			queue_work(dm_bufio_wq, &dm_bufio_replacement_work);
 	} else {
 		list_del(&b->global_list);
+		global_num--;
 	}
 
 	spin_unlock(&global_spinlock);
@@ -497,6 +513,8 @@ static void __relink_lru(struct dm_buffe
 {
 	struct dm_bufio_client *c = b->c;
 
+	b->accessed = 1;
+
 	BUG_ON(!c->n_buffers[b->list_mode]);
 
 	c->n_buffers[b->list_mode]--;
@@ -1814,6 +1832,78 @@ static void __evict_old_buffers(struct d
 	dm_bufio_unlock(c);
 }
 
+static void do_global_cleanup(struct work_struct *w)
+{
+	struct dm_bufio_client *locked_client = NULL;
+	struct dm_bufio_client *current_client;
+	struct dm_buffer *b;
+	unsigned spinlock_hold_count;
+	unsigned long threshold = dm_bufio_cache_size - dm_bufio_cache_size / DM_BUFIO_LOW_WATERMARK;
+	unsigned long loops = global_num * 2;
+
+	mutex_lock(&dm_bufio_clients_lock);
+
+reacquire_spinlock:
+	cond_resched();
+
+	spin_lock(&global_spinlock);
+	if (unlikely(dm_bufio_current_allocated <= threshold))
+		goto exit;
+
+	spinlock_hold_count = 0;
+get_next:
+	if (!loops--)
+		goto exit;
+	if (unlikely(list_empty(&global_queue)))
+		goto exit;
+	b = list_entry(global_queue.prev, struct dm_buffer, global_list);
+
+	if (b->accessed) {
+		b->accessed = 0;
+		list_del(&b->global_list);
+		list_add(&b->global_list, &global_queue);
+		if (likely(++spinlock_hold_count < 16)) {
+			goto get_next;
+		}
+		spin_unlock(&global_spinlock);
+		goto reacquire_spinlock;
+	}
+
+	current_client = b->c;
+	if (unlikely(current_client != locked_client)) {
+		if (locked_client)
+			dm_bufio_unlock(locked_client);
+
+		if (!dm_bufio_trylock(current_client)) {
+			spin_unlock(&global_spinlock);
+			dm_bufio_lock(current_client);
+			locked_client = current_client;
+			goto reacquire_spinlock;
+		}
+
+		locked_client = current_client;
+	}
+
+	spin_unlock(&global_spinlock);
+
+	if (unlikely(!__try_evict_buffer(b, GFP_KERNEL))) {
+		spin_lock(&global_spinlock);
+		list_del(&b->global_list);
+		list_add(&b->global_list, &global_queue);
+		spin_unlock(&global_spinlock);
+	}
+
+	goto reacquire_spinlock;
+
+exit:
+	spin_unlock(&global_spinlock);
+
+	if (locked_client)
+		dm_bufio_unlock(locked_client);
+
+	mutex_unlock(&dm_bufio_clients_lock);
+}
+
 static void cleanup_old_buffers(void)
 {
 	unsigned long max_age_hz = get_max_age_hz();
@@ -1829,14 +1919,11 @@ static void cleanup_old_buffers(void)
 	mutex_unlock(&dm_bufio_clients_lock);
 }
 
-static struct workqueue_struct *dm_bufio_wq;
-static struct delayed_work dm_bufio_work;
-
 static void work_fn(struct work_struct *w)
 {
 	cleanup_old_buffers();
 
-	queue_delayed_work(dm_bufio_wq, &dm_bufio_work,
+	queue_delayed_work(dm_bufio_wq, &dm_bufio_cleanup_old_work,
 			   DM_BUFIO_WORK_TIMER_SECS * HZ);
 }
 
@@ -1878,8 +1965,9 @@ static int __init dm_bufio_init(void)
 	if (!dm_bufio_wq)
 		return -ENOMEM;
 
-	INIT_DELAYED_WORK(&dm_bufio_work, work_fn);
-	queue_delayed_work(dm_bufio_wq, &dm_bufio_work,
+	INIT_DELAYED_WORK(&dm_bufio_cleanup_old_work, work_fn);
+	INIT_WORK(&dm_bufio_replacement_work, do_global_cleanup);
+	queue_delayed_work(dm_bufio_wq, &dm_bufio_cleanup_old_work,
 			   DM_BUFIO_WORK_TIMER_SECS * HZ);
 
 	return 0;
@@ -1892,7 +1980,8 @@ static void __exit dm_bufio_exit(void)
 {
 	int bug = 0;
 
-	cancel_delayed_work_sync(&dm_bufio_work);
+	cancel_delayed_work_sync(&dm_bufio_cleanup_old_work);
+	flush_workqueue(dm_bufio_wq);
 	destroy_workqueue(dm_bufio_wq);
 
 	if (dm_bufio_client_count) {