[PATCH 1/1] apparmor: cache buffers on percpu list if there is lock contention

[John Johansen]

On 2/16/23 15:42, John Johansen wrote:
> On 2/16/23 13:46, Anil Altinay wrote:
>> 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.
>>
>> Fixes: df323337e507 ("apparmor: Use a memory pool instead per-CPU caches")
>> Link: https://lore.kernel.org/lkml/cfd5cc6f-5943-2e06-1dbe-f4b4ad5c1fa1@canonical.com/
>> Signed-off-by: John Johansen <john.johansen at canonical.com>
>> Reported-by: Sergey Senozhatsky <senozhatsky at chromium.org>
>> Signed-off-by: Anil Altinay <aaltinay at google.com>
>> Cc: stable at vger.kernel.org
> 
> NAK, this version of the patch has an issue that prevented it from
> being pushed upstream.
> 
> I can send out the revised version for people to look at but I
> haven't been able to get proper testing on it yet, hence why
> I haven't pushed it yet either.
> 

I should add what we need to get this upstream is some testing. I
think its good but I haven't been able do any testing of v3 on
a larger machine.

There is a debug patch below that can be dropped on top of v3. Its
not included as part of the patch series as it needs some
revisions/improvements if its going to go upstream but its works
for devel debugging.


commit f44dee132b0b55386b7ea31e68c80d367b073ee0
Author: John Johansen <john.johansen at canonical.com>
Date:   Tue Oct 25 01:18:41 2022 -0700

     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>

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