[PATCH v3] apparmor: global buffers spin lock may get contended

[John Johansen]

 From f44dee132b0b55386b7ea31e68c80d367b073ee0 Mon Sep 17 00:00:00 2001
From: John Johansen <john.johansen at canonical.com>
Date: Tue, 25 Oct 2022 01:18:41 -0700
Subject: [PATCH] apparmor: cache buffers on percpu list if there is lock
  contention

On a heavily loaded machine there can be lock contention on the
global buffers lock. Add a percpu list to cache buffers on when
lock contention is encountered.

When allocating buffers attempt to use cached buffers first,
before taking the global buffers lock. When freeing buffers
try to put them back to the global list but if contention is
encountered, put the buffer on the percpu list.

The length of time a buffer is held on the percpu list is dynamically
adjusted based on lock contention.  The amount of hold time is rapidly
increased and slow ramped down.

v3:
- limit number of buffers that can be pushed onto the percpu
   list. This avoids a problem on some kernels where one percpu
   list can inherit buffers from another cpu after a reschedule,
   causing more kernel memory to used than is necessary. Under
   normal conditions this should eventually return to normal
   but under pathelogical conditions the extra memory consumption
   may have been unbouanded
v2:
- dynamically adjust buffer hold time on percpu list based on
   lock contention.
v1:
- cache buffers on percpu list on lock contention

Signed-off-by: John Johansen <john.johansen at canonical.com>
---
  security/apparmor/lsm.c | 81 ++++++++++++++++++++++++++++++++++++++---
  1 file changed, 76 insertions(+), 5 deletions(-)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 25114735bc11..21f5ea20e715 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -49,12 +49,19 @@ union aa_buffer {
  	char buffer[1];
  };
  
+struct aa_local_cache {
+	unsigned int contention;
+	unsigned int hold;
+	struct list_head head;
+};
+
  #define RESERVE_COUNT 2
  static int reserve_count = RESERVE_COUNT;
  static int buffer_count;
  
  static LIST_HEAD(aa_global_buffers);
  static DEFINE_SPINLOCK(aa_buffers_lock);
+static DEFINE_PER_CPU(struct aa_local_cache, aa_local_buffers);
  
  /*
   * LSM hook functions
@@ -1622,14 +1629,44 @@ static int param_set_mode(const char *val, const struct kernel_param *kp)
  	return 0;
  }
  
+static void update_contention(struct aa_local_cache *cache)
+{
+	cache->contention += 3;
+	if (cache->contention > 9)
+		cache->contention = 9;
+	cache->hold += 1 << cache->contention;		/* 8, 64, 512 */
+}
+
  char *aa_get_buffer(bool in_atomic)
  {
  	union aa_buffer *aa_buf;
+	struct aa_local_cache *cache;
  	bool try_again = true;
  	gfp_t flags = (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
  
+	/* use per cpu cached buffers first */
+	cache = get_cpu_ptr(&aa_local_buffers);
+	if (!list_empty(&cache->head)) {
+		aa_buf = list_first_entry(&cache->head, union aa_buffer, list);
+		list_del(&aa_buf->list);
+		cache->hold--;
+		put_cpu_ptr(&aa_local_buffers);
+		return &aa_buf->buffer[0];
+	}
+	put_cpu_ptr(&aa_local_buffers);
+
+	if (!spin_trylock(&aa_buffers_lock)) {
+		cache = get_cpu_ptr(&aa_local_buffers);
+		update_contention(cache);
+		put_cpu_ptr(&aa_local_buffers);
+		spin_lock(&aa_buffers_lock);
+	} else {
+		cache = get_cpu_ptr(&aa_local_buffers);
+		if (cache->contention)
+			cache->contention--;
+		put_cpu_ptr(&aa_local_buffers);
+	}
  retry:
-	spin_lock(&aa_buffers_lock);
  	if (buffer_count > reserve_count ||
  	    (in_atomic && !list_empty(&aa_global_buffers))) {
  		aa_buf = list_first_entry(&aa_global_buffers, union aa_buffer,
@@ -1655,6 +1692,7 @@ char *aa_get_buffer(bool in_atomic)
  	if (!aa_buf) {
  		if (try_again) {
  			try_again = false;
+			spin_lock(&aa_buffers_lock);
  			goto retry;
  		}
  		pr_warn_once("AppArmor: Failed to allocate a memory buffer.\n");
@@ -1666,15 +1704,39 @@ char *aa_get_buffer(bool in_atomic)
  void aa_put_buffer(char *buf)
  {
  	union aa_buffer *aa_buf;
+	struct aa_local_cache *cache;
  
  	if (!buf)
  		return;
  	aa_buf = container_of(buf, union aa_buffer, buffer[0]);
  
-	spin_lock(&aa_buffers_lock);
-	list_add(&aa_buf->list, &aa_global_buffers);
-	buffer_count++;
-	spin_unlock(&aa_buffers_lock);
+	cache = get_cpu_ptr(&aa_local_buffers);
+	if (!cache->hold || cache->count >= 2) {
+		put_cpu_ptr(&aa_local_buffers);
+		if (spin_trylock(&aa_buffers_lock)) {
+		locked:
+			list_add(&aa_buf->list, &aa_global_buffers);
+			buffer_count++;
+			spin_unlock(&aa_buffers_lock);
+			cache = get_cpu_ptr(&aa_local_buffers);
+			if (cache->contention)
+				cache->contention--;
+			put_cpu_ptr(&aa_local_buffers);
+			return;
+		}
+		cache = get_cpu_ptr(&aa_local_buffers);
+		update_contention(cache);
+		if (cache->count >= 2) {
+			put_cpu_ptr(&aa_local_buffers);
+			spin_lock(&aa_buffers_lock);
+			/* force putting the buffer to global */
+			goto locked;
+		}
+	}
+
+	/* cache in percpu list */
+	list_add(&aa_buf->list, &cache->head);
+	put_cpu_ptr(&aa_local_buffers);
  }
  
  /*
@@ -1716,6 +1778,15 @@ static int __init alloc_buffers(void)
  	union aa_buffer *aa_buf;
  	int i, num;
  
+	/*
+	 * per cpu set of cached allocated buffers used to help reduce
+	 * lock contention
+	 */
+	for_each_possible_cpu(i) {
+		per_cpu(aa_local_buffers, i).contention = 0;
+		per_cpu(aa_local_buffers, i).hold = 0;
+		INIT_LIST_HEAD(&per_cpu(aa_local_buffers, i).head);
+	}
  	/*
  	 * A function may require two buffers at once. Usually the buffers are
  	 * used for a short period of time and are shared. On UP kernel buffers
-- 
2.34.1