apparmor: global buffers spin lock may get contended

apparmor: global buffers spin lock may get contended

[Sergey Senozhatsky]

Hi,

We've notices that apparmor has switched from using per-CPU buffer pool
and per-CPU spin_lock to a global spin_lock in df323337e507a0009d3db1ea.

This seems to be causing some contention on our build machines (with
quite a bit of cores). Because that global spin lock is a part of the
stat() sys call (and perhaps some other)

E.g.

-    9.29%     0.00%  clang++          [kernel.vmlinux]                        
   - 9.28% entry_SYSCALL_64_after_hwframe                                      
      - 8.98% do_syscall_64                                                    
         - 7.43% __do_sys_newlstat                                            
            - 7.43% vfs_statx                                                  
               - 7.18% security_inode_getattr                                  
                  - 7.15% apparmor_inode_getattr                              
                     - aa_path_perm                                            
                        - 3.53% aa_get_buffer                                  
                           - 3.47% _raw_spin_lock                              
                                3.44% native_queued_spin_lock_slowpath        
                        - 3.49% aa_put_buffer.part.0                          
                           - 3.45% _raw_spin_lock                              
                                3.43% native_queued_spin_lock_slowpath   

Can we fix this contention?

Re: apparmor: global buffers spin lock may get contended

[John Johansen]

On 7/13/21 6:19 AM, Sergey Senozhatsky wrote:
> Hi,
> 
> We've notices that apparmor has switched from using per-CPU buffer pool
> and per-CPU spin_lock to a global spin_lock in df323337e507a0009d3db1ea.
> 
> This seems to be causing some contention on our build machines (with
> quite a bit of cores). Because that global spin lock is a part of the
> stat() sys call (and perhaps some other)
> 
> E.g.
> 
> -    9.29%     0.00%  clang++          [kernel.vmlinux]                        
>    - 9.28% entry_SYSCALL_64_after_hwframe                                      
>       - 8.98% do_syscall_64                                                    
>          - 7.43% __do_sys_newlstat                                            
>             - 7.43% vfs_statx                                                  
>                - 7.18% security_inode_getattr                                  
>                   - 7.15% apparmor_inode_getattr                              
>                      - aa_path_perm                                            
>                         - 3.53% aa_get_buffer                                  
>                            - 3.47% _raw_spin_lock                              
>                                 3.44% native_queued_spin_lock_slowpath        
>                         - 3.49% aa_put_buffer.part.0                          
>                            - 3.45% _raw_spin_lock                              
>                                 3.43% native_queued_spin_lock_slowpath   
> 
> Can we fix this contention?
> 

sorry this got filtered to a wrong mailbox. Yes this is something that can
be improved, and was a concern when the switch was made from per-CPU buffers
to the global pool.

We can look into doing a hybrid approach where we can per cpu cache a buffer
from the global pool. The trick will be coming up with when the cached buffer
can be returned so we don't run into the problems that lead to
df323337e507a0009d3db1ea

Re: apparmor: global buffers spin lock may get contended

[John Johansen]

On 7/13/21 06:19, Sergey Senozhatsky wrote:
> Hi,
> 
> We've notices that apparmor has switched from using per-CPU buffer pool
> and per-CPU spin_lock to a global spin_lock in df323337e507a0009d3db1ea.
> 
> This seems to be causing some contention on our build machines (with
> quite a bit of cores). Because that global spin lock is a part of the
> stat() sys call (and perhaps some other)
> 
> E.g.
> 
> -    9.29%     0.00%  clang++          [kernel.vmlinux]
>     - 9.28% entry_SYSCALL_64_after_hwframe
>        - 8.98% do_syscall_64
>           - 7.43% __do_sys_newlstat
>              - 7.43% vfs_statx
>                 - 7.18% security_inode_getattr
>                    - 7.15% apparmor_inode_getattr
>                       - aa_path_perm
>                          - 3.53% aa_get_buffer
>                             - 3.47% _raw_spin_lock
>                                  3.44% native_queued_spin_lock_slowpath
>                          - 3.49% aa_put_buffer.part.0
>                             - 3.45% _raw_spin_lock
>                                  3.43% native_queued_spin_lock_slowpath
> 
> Can we fix this contention?

sorry for the delay on this. Below is a proposed patch that I have been testing
to deal with this issue.


 From d026988196fdbda7234fb87bc3e4aea22edcbaf9 Mon Sep 17 00:00:00 2001
From: John Johansen <john.johansen@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.

Signed-off-by: John Johansen <john.johansen@canonical.com>
---
  security/apparmor/lsm.c | 74 ++++++++++++++++++++++++++++++++++++++---
  1 file changed, 69 insertions(+), 5 deletions(-)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 25114735bc11..0ab70171bdb6 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 +1704,32 @@ 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) {
+		put_cpu_ptr(&aa_local_buffers);
+		if (spin_trylock(&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->contention)
+				cache->contention--;
+			put_cpu_ptr(&aa_local_buffers);
+			return;
+		}
+		cache = get_cpu_ptr(&aa_local_buffers);
+		update_contention(cache);
+	}
+
+	/* cache in percpu list */
+	list_add(&aa_buf->list, &cache->head);
+	put_cpu_ptr(&aa_local_buffers);
  }
  
  /*
@@ -1716,6 +1771,15 @@ 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

Re: apparmor: global buffers spin lock may get contended

[Sergey Senozhatsky]

On (22/10/28 02:34), John Johansen wrote:
> From d026988196fdbda7234fb87bc3e4aea22edcbaf9 Mon Sep 17 00:00:00 2001
> From: John Johansen <john.johansen@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.
> 
> Signed-off-by: John Johansen <john.johansen@canonical.com>

Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Re: apparmor: global buffers spin lock may get contended

[John Johansen]

On 10/30/22 20:52, Sergey Senozhatsky wrote:
> On (22/10/28 02:34), John Johansen wrote:
>>  From d026988196fdbda7234fb87bc3e4aea22edcbaf9 Mon Sep 17 00:00:00 2001
>> From: John Johansen <john.johansen@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.
>>
>> Signed-off-by: John Johansen <john.johansen@canonical.com>
> 
> Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>

yep, thanks for catching that

Re: apparmor: global buffers spin lock may get contended

[Sergey Senozhatsky]

On (22/10/30 20:55), John Johansen wrote:
> On 10/30/22 20:52, Sergey Senozhatsky wrote:
> > On (22/10/28 02:34), John Johansen wrote:
> > >  From d026988196fdbda7234fb87bc3e4aea22edcbaf9 Mon Sep 17 00:00:00 2001
> > > From: John Johansen <john.johansen@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.
> > > 
> > > Signed-off-by: John Johansen <john.johansen@canonical.com>
> > 
> > Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> 
> yep, thanks for catching that

Thanks for the patch! Unfortunately it'll be a bit difficult to test
it right now; I'll probably have to wait until corp pushes new kernel
(with the patch) to build boxes.

Re: apparmor: global buffers spin lock may get contended

[John Johansen]

I have sent up a new version of this patch that caps a situation where buffer lists could grow unbounded (at theoretically).

[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@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@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

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

[Sebastian Andrzej Siewior]

On 2023-02-16 16:08:10 [-0800], John Johansen wrote:
> --- 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;
> +};

if you stick a local_lock_t into that struct, then you could replace
	cache = get_cpu_ptr(&aa_local_buffers);
with
	local_lock(&aa_local_buffers.lock);
	cache = this_cpu_ptr(&aa_local_buffers);

You would get the preempt_disable() based locking for the per-CPU
variable (as with get_cpu_ptr()) and additionally some lockdep
validation which would warn if it is used outside of task context (IRQ).

I didn't parse completely the hold/contention logic but it seems to work
;)
You check "cache->count >=  2" twice but I don't see an inc/ dec of it
nor is it part of aa_local_cache.

I can't parse how many items can end up on the local list if the global
list is locked. My guess would be more than 2 due the ->hold parameter.

Do you have any numbers on the machine and performance it improved? It
sure will be a good selling point.

Sebastian

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

[John Johansen]

On 2/17/23 02:44, Sebastian Andrzej Siewior wrote:
> On 2023-02-16 16:08:10 [-0800], John Johansen wrote:
>> --- 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;
>> +};
> 
> if you stick a local_lock_t into that struct, then you could replace
> 	cache = get_cpu_ptr(&aa_local_buffers);
> with
> 	local_lock(&aa_local_buffers.lock);
> 	cache = this_cpu_ptr(&aa_local_buffers);
> 
> You would get the preempt_disable() based locking for the per-CPU
> variable (as with get_cpu_ptr()) and additionally some lockdep
> validation which would warn if it is used outside of task context (IRQ).
> 
I did look at local_locks and there was a reason I didn't use them. I
can't recall as the original iteration of this is over a year old now.
I will have to dig into it again.

> I didn't parse completely the hold/contention logic but it seems to work
> ;)
> You check "cache->count >=  2" twice but I don't see an inc/ dec of it
> nor is it part of aa_local_cache.
> 
sadly I messed up the reordering of this and the debug patch. This will be
fixed in v4.

> I can't parse how many items can end up on the local list if the global
> list is locked. My guess would be more than 2 due the ->hold parameter.
> 
So this iteration, forces pushing back to global list if there are already
two on the local list. The hold parameter just affects how long the
buffers remain on the local list, before trying to place them back on
the global list.

Originally before the count was added more than 2 buffers could end up
on the local list, and having too many local buffers is a waste of
memory. The count got added to address this. The value of 2 (which should
be switched to a define) was chosen because no mediation routine currently
uses more than 2 buffers.

Note that this doesn't mean that more than two buffers can be allocated
to a tasks on a cpu. Its possible in some cases to have a task have
allocated buffers and to still have buffers on the local cache list.

> Do you have any numbers on the machine and performance it improved? It
> sure will be a good selling point.
> 

I can include some supporting info, for a 16 core machine. But it will
take some time to for me to get access to a bigger machine, where this
is much more important. Hence the call for some of the other people
on this thread to test.

thanks for the feedback

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

[Anil Altinay]

I can test the patch with 5.10 and 5.15 kernels in different machines.
Just let me know which machine types you would like me to test.

On Mon, Feb 20, 2023 at 12:42 AM John Johansen
<john.johansen@canonical.com> wrote:
>
> On 2/17/23 02:44, Sebastian Andrzej Siewior wrote:
> > On 2023-02-16 16:08:10 [-0800], John Johansen wrote:
> >> --- 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;
> >> +};
> >
> > if you stick a local_lock_t into that struct, then you could replace
> >       cache = get_cpu_ptr(&aa_local_buffers);
> > with
> >       local_lock(&aa_local_buffers.lock);
> >       cache = this_cpu_ptr(&aa_local_buffers);
> >
> > You would get the preempt_disable() based locking for the per-CPU
> > variable (as with get_cpu_ptr()) and additionally some lockdep
> > validation which would warn if it is used outside of task context (IRQ).
> >
> I did look at local_locks and there was a reason I didn't use them. I
> can't recall as the original iteration of this is over a year old now.
> I will have to dig into it again.
>
> > I didn't parse completely the hold/contention logic but it seems to work
> > ;)
> > You check "cache->count >=  2" twice but I don't see an inc/ dec of it
> > nor is it part of aa_local_cache.
> >
> sadly I messed up the reordering of this and the debug patch. This will be
> fixed in v4.
>
> > I can't parse how many items can end up on the local list if the global
> > list is locked. My guess would be more than 2 due the ->hold parameter.
> >
> So this iteration, forces pushing back to global list if there are already
> two on the local list. The hold parameter just affects how long the
> buffers remain on the local list, before trying to place them back on
> the global list.
>
> Originally before the count was added more than 2 buffers could end up
> on the local list, and having too many local buffers is a waste of
> memory. The count got added to address this. The value of 2 (which should
> be switched to a define) was chosen because no mediation routine currently
> uses more than 2 buffers.
>
> Note that this doesn't mean that more than two buffers can be allocated
> to a tasks on a cpu. Its possible in some cases to have a task have
> allocated buffers and to still have buffers on the local cache list.
>
> > Do you have any numbers on the machine and performance it improved? It
> > sure will be a good selling point.
> >
>
> I can include some supporting info, for a 16 core machine. But it will
> take some time to for me to get access to a bigger machine, where this
> is much more important. Hence the call for some of the other people
> on this thread to test.
>
> thanks for the feedback
>

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

[Anil Altinay]

Hi John,

I was wondering if you get a chance to work on patch v4. Please let me
know if you need help with testing.

Best,
Anil


On Tue, Feb 21, 2023 at 1:27 PM Anil Altinay <aaltinay@google.com> wrote:
>
> I can test the patch with 5.10 and 5.15 kernels in different machines.
> Just let me know which machine types you would like me to test.
>
> On Mon, Feb 20, 2023 at 12:42 AM John Johansen
> <john.johansen@canonical.com> wrote:
> >
> > On 2/17/23 02:44, Sebastian Andrzej Siewior wrote:
> > > On 2023-02-16 16:08:10 [-0800], John Johansen wrote:
> > >> --- 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;
> > >> +};
> > >
> > > if you stick a local_lock_t into that struct, then you could replace
> > >       cache = get_cpu_ptr(&aa_local_buffers);
> > > with
> > >       local_lock(&aa_local_buffers.lock);
> > >       cache = this_cpu_ptr(&aa_local_buffers);
> > >
> > > You would get the preempt_disable() based locking for the per-CPU
> > > variable (as with get_cpu_ptr()) and additionally some lockdep
> > > validation which would warn if it is used outside of task context (IRQ).
> > >
> > I did look at local_locks and there was a reason I didn't use them. I
> > can't recall as the original iteration of this is over a year old now.
> > I will have to dig into it again.
> >
> > > I didn't parse completely the hold/contention logic but it seems to work
> > > ;)
> > > You check "cache->count >=  2" twice but I don't see an inc/ dec of it
> > > nor is it part of aa_local_cache.
> > >
> > sadly I messed up the reordering of this and the debug patch. This will be
> > fixed in v4.
> >
> > > I can't parse how many items can end up on the local list if the global
> > > list is locked. My guess would be more than 2 due the ->hold parameter.
> > >
> > So this iteration, forces pushing back to global list if there are already
> > two on the local list. The hold parameter just affects how long the
> > buffers remain on the local list, before trying to place them back on
> > the global list.
> >
> > Originally before the count was added more than 2 buffers could end up
> > on the local list, and having too many local buffers is a waste of
> > memory. The count got added to address this. The value of 2 (which should
> > be switched to a define) was chosen because no mediation routine currently
> > uses more than 2 buffers.
> >
> > Note that this doesn't mean that more than two buffers can be allocated
> > to a tasks on a cpu. Its possible in some cases to have a task have
> > allocated buffers and to still have buffers on the local cache list.
> >
> > > Do you have any numbers on the machine and performance it improved? It
> > > sure will be a good selling point.
> > >
> >
> > I can include some supporting info, for a 16 core machine. But it will
> > take some time to for me to get access to a bigger machine, where this
> > is much more important. Hence the call for some of the other people
> > on this thread to test.
> >
> > thanks for the feedback
> >

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

[John Johansen]

On 6/26/23 16:33, Anil Altinay wrote:
> Hi John,
> 
> I was wondering if you get a chance to work on patch v4. Please let me know if you need help with testing.
> 

yeah, testing help is always much appreciated. I have a v4, and I am working on 3 alternate version to compare against, to help give a better sense if we can get away with simplifying or tweak the scaling. I should be able to post them out some time tonight.

> Best,
> Anil
> 
> On Tue, Feb 21, 2023 at 1:27 PM Anil Altinay <aaltinay@google.com <mailto:aaltinay@google.com>> wrote:
> 
>     I can test the patch with 5.10 and 5.15 kernels in different machines.
>     Just let me know which machine types you would like me to test.
> 
>     On Mon, Feb 20, 2023 at 12:42 AM John Johansen
>     <john.johansen@canonical.com <mailto:john.johansen@canonical.com>> wrote:
>      >
>      > On 2/17/23 02:44, Sebastian Andrzej Siewior wrote:
>      > > On 2023-02-16 16:08:10 [-0800], John Johansen wrote:
>      > >> --- 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;
>      > >> +};
>      > >
>      > > if you stick a local_lock_t into that struct, then you could replace
>      > >       cache = get_cpu_ptr(&aa_local_buffers);
>      > > with
>      > >       local_lock(&aa_local_buffers.lock);
>      > >       cache = this_cpu_ptr(&aa_local_buffers);
>      > >
>      > > You would get the preempt_disable() based locking for the per-CPU
>      > > variable (as with get_cpu_ptr()) and additionally some lockdep
>      > > validation which would warn if it is used outside of task context (IRQ).
>      > >
>      > I did look at local_locks and there was a reason I didn't use them. I
>      > can't recall as the original iteration of this is over a year old now.
>      > I will have to dig into it again.
>      >
>      > > I didn't parse completely the hold/contention logic but it seems to work
>      > > ;)
>      > > You check "cache->count >=  2" twice but I don't see an inc/ dec of it
>      > > nor is it part of aa_local_cache.
>      > >
>      > sadly I messed up the reordering of this and the debug patch. This will be
>      > fixed in v4.
>      >
>      > > I can't parse how many items can end up on the local list if the global
>      > > list is locked. My guess would be more than 2 due the ->hold parameter.
>      > >
>      > So this iteration, forces pushing back to global list if there are already
>      > two on the local list. The hold parameter just affects how long the
>      > buffers remain on the local list, before trying to place them back on
>      > the global list.
>      >
>      > Originally before the count was added more than 2 buffers could end up
>      > on the local list, and having too many local buffers is a waste of
>      > memory. The count got added to address this. The value of 2 (which should
>      > be switched to a define) was chosen because no mediation routine currently
>      > uses more than 2 buffers.
>      >
>      > Note that this doesn't mean that more than two buffers can be allocated
>      > to a tasks on a cpu. Its possible in some cases to have a task have
>      > allocated buffers and to still have buffers on the local cache list.
>      >
>      > > Do you have any numbers on the machine and performance it improved? It
>      > > sure will be a good selling point.
>      > >
>      >
>      > I can include some supporting info, for a 16 core machine. But it will
>      > take some time to for me to get access to a bigger machine, where this
>      > is much more important. Hence the call for some of the other people
>      > on this thread to test.
>      >
>      > thanks for the feedback
>      >
> 

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

[Sergey Senozhatsky]

On (23/06/26 17:31), John Johansen wrote:
> On 6/26/23 16:33, Anil Altinay wrote:
> > Hi John,
> > 
> > I was wondering if you get a chance to work on patch v4. Please let me know if you need help with testing.
> > 
> 
> yeah, testing help is always much appreciated. I have a v4, and I am
> working on 3 alternate version to compare against, to help give a better
> sense if we can get away with simplifying or tweak the scaling.
>
> I should be able to post them out some time tonight.

Hi John,

Did you get a chance to post v4? I may be able to give it some testing
on our real-life case.

[PATCH v5 0/4] apparmor: cache buffers on percpu list if there is lock, contention

[John Johansen]

On 10/5/23 21:18, Sergey Senozhatsky wrote:
> On (23/06/26 17:31), John Johansen wrote:
>> On 6/26/23 16:33, Anil Altinay wrote:
>>> Hi John,
>>>
>>> I was wondering if you get a chance to work on patch v4. Please let me know if you need help with testing.
>>>
>>
>> yeah, testing help is always much appreciated. I have a v4, and I am
>> working on 3 alternate version to compare against, to help give a better
>> sense if we can get away with simplifying or tweak the scaling.
>>
>> I should be able to post them out some time tonight.
> 
> Hi John,
> 
> Did you get a chance to post v4? I may be able to give it some testing
> on our real-life case.

sorry yes, how about a v5. That is simplified with 3 follow on patches
that aren't strictly necessary, but some combination of them might be
better than just the base patch, but splitting them out makes the
individual changes easier to review.

---


df323337e507 ("apparmor: Use a memory pool instead per-CPU caches")
changed buffer allocation to use a memory pool, however 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.

v5:
- simplify base patch by removing: improvements can be added later
   - MAX_LOCAL and must lock
   - contention scaling.
v4:
- fix percpu ->count buffer count which had been spliced across a
   debug patch.
- introduce define for MAX_LOCAL_COUNT
- rework count check and locking around it.
- update commit message to reference commit that introduced the
   memory.
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

Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Re: [PATCH v5 1/4] apparmor: cache buffers on percpu list if there is lock, contention

[John Johansen]

df323337e507 ("apparmor: Use a memory pool instead per-CPU caches")
changed buffer allocation to use a memory pool, however 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
increased and decreased linearly.

Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: John Johansen <john.johansen@canonical.com>
---
  security/apparmor/lsm.c | 67 ++++++++++++++++++++++++++++++++++++++---
  1 file changed, 62 insertions(+), 5 deletions(-)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index c80c1bd3024a..ce4f3e7a784d 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -49,12 +49,19 @@ union aa_buffer {
  	DECLARE_FLEX_ARRAY(char, buffer);
  };
  
+struct aa_local_cache {
+	unsigned int hold;
+	unsigned int count;
+	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
@@ -1789,11 +1796,32 @@ static int param_set_mode(const char *val, const struct kernel_param *kp)
  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--;
+		cache->count--;
+		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);
+		cache->hold += 1;
+		put_cpu_ptr(&aa_local_buffers);
+		spin_lock(&aa_buffers_lock);
+	} else {
+		cache = get_cpu_ptr(&aa_local_buffers);
+		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,
@@ -1819,6 +1847,7 @@ 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");
@@ -1830,15 +1859,34 @@ 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) {
+		put_cpu_ptr(&aa_local_buffers);
+
+		if (spin_trylock(&aa_buffers_lock)) {
+			/* put back on global list */
+			list_add(&aa_buf->list, &aa_global_buffers);
+			buffer_count++;
+			spin_unlock(&aa_buffers_lock);
+			cache = get_cpu_ptr(&aa_local_buffers);
+			put_cpu_ptr(&aa_local_buffers);
+			return;
+		}
+		/* contention on global list, fallback to percpu */
+		cache = get_cpu_ptr(&aa_local_buffers);
+		cache->hold += 1;
+	}
+
+	/* cache in percpu list */
+	list_add(&aa_buf->list, &cache->head);
+	cache->count++;
+	put_cpu_ptr(&aa_local_buffers);
  }
  
  /*
@@ -1880,6 +1928,15 @@ 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).hold = 0;
+		per_cpu(aa_local_buffers, i).count = 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

Re: [PATCH v5 2/4] apparmor: exponential backoff on cache buffer contention

[John Johansen]

Reduce contention on the global buffers lock by using  an exponential
back off strategy where the amount tries to hold is doubled when
contention is encoutered and backed off linearly when there isn't
contention.

Signed-off-by: John Johansen <john.johansen@canonical.com>
---
  security/apparmor/lsm.c | 18 ++++++++++++++++--
  1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index ce4f3e7a784d..fd6779ff0da4 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -50,6 +50,7 @@ union aa_buffer {
  };
  
  struct aa_local_cache {
+	unsigned int contention;
  	unsigned int hold;
  	unsigned int count;
  	struct list_head head;
@@ -1793,6 +1794,14 @@ 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 += 1;
+	if (cache->contention > 9)
+		cache->contention = 9;
+	cache->hold += 1 << cache->contention;		/* 2, 4, 8, ... */
+}
+
  char *aa_get_buffer(bool in_atomic)
  {
  	union aa_buffer *aa_buf;
@@ -1814,11 +1823,13 @@ char *aa_get_buffer(bool in_atomic)
  
  	if (!spin_trylock(&aa_buffers_lock)) {
  		cache = get_cpu_ptr(&aa_local_buffers);
-		cache->hold += 1;
+		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:
@@ -1875,12 +1886,14 @@ void aa_put_buffer(char *buf)
  			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;
  		}
  		/* contention on global list, fallback to percpu */
  		cache = get_cpu_ptr(&aa_local_buffers);
-		cache->hold += 1;
+		update_contention(cache);
  	}
  
  	/* cache in percpu list */
@@ -1933,6 +1946,7 @@ static int __init alloc_buffers(void)
  	 * lock contention
  	 */
  	for_each_possible_cpu(i) {
+		per_cpu(aa_local_buffers, i).contention = 0;
  		per_cpu(aa_local_buffers, i).hold = 0;
  		per_cpu(aa_local_buffers, i).count = 0;
  		INIT_LIST_HEAD(&per_cpu(aa_local_buffers, i).head);
-- 
2.34.1

Re: [PATCH v5 3/4] apparmor: experiment with faster backoff on global buffer

[John Johansen]

Instead of doubling hold count when contention is encounter increase
it by 8x. This makes for a faster back off, but results in buffers
being held longer.

Signed-off-by: John Johansen <john.johansen@canonical.com>
---
  security/apparmor/lsm.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index fd6779ff0da4..52423d88854a 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -1796,10 +1796,10 @@ static int param_set_mode(const char *val, const struct kernel_param *kp)
  
  static void update_contention(struct aa_local_cache *cache)
  {
-	cache->contention += 1;
+	cache->contention += 3;
  	if (cache->contention > 9)
  		cache->contention = 9;
-	cache->hold += 1 << cache->contention;		/* 2, 4, 8, ... */
+	cache->hold += 1 << cache->contention;		/* 8, 64, 512, ... */
  }
  
  char *aa_get_buffer(bool in_atomic)
-- 
2.34.1

Re: [PATCH v5 4/4] apparmor: limit the number of buffers in percpu cache

[John Johansen]

Force buffers to be returned to the global pool, regardless of contention
when the percpu cache is full. This ensures that the percpu buffer list
never grows longer than needed.

Signed-off-by: John Johansen <john.johansen@canonical.com>
---
  security/apparmor/lsm.c | 9 ++++++++-
  1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 52423d88854a..e6765f64f6bf 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -56,6 +56,7 @@ struct aa_local_cache {
  	struct list_head head;
  };
  
+#define MAX_LOCAL_COUNT 2
  #define RESERVE_COUNT 2
  static int reserve_count = RESERVE_COUNT;
  static int buffer_count;
@@ -1878,9 +1879,15 @@ void aa_put_buffer(char *buf)
  
  	cache = get_cpu_ptr(&aa_local_buffers);
  	if (!cache->hold) {
+		bool must_lock = cache->count >= MAX_LOCAL_COUNT;
+
  		put_cpu_ptr(&aa_local_buffers);
  
-		if (spin_trylock(&aa_buffers_lock)) {
+		if (must_lock) {
+			spin_lock(&aa_buffers_lock);
+			goto locked;
+		} else if (spin_trylock(&aa_buffers_lock)) {
+		locked:
  			/* put back on global list */
  			list_add(&aa_buf->list, &aa_global_buffers);
  			buffer_count++;
-- 
2.34.1

Re: [PATCH v5 0/4] apparmor: cache buffers on percpu list if there is lock, contention

[Sergey Senozhatsky]

On (23/10/17 02:21), John Johansen wrote:
> > > yeah, testing help is always much appreciated. I have a v4, and I am
> > > working on 3 alternate version to compare against, to help give a better
> > > sense if we can get away with simplifying or tweak the scaling.
> > > 
> > > I should be able to post them out some time tonight.
> > 
> > Hi John,
> > 
> > Did you get a chance to post v4? I may be able to give it some testing
> > on our real-life case.
> 
> sorry yes, how about a v5. That is simplified with 3 follow on patches
> that aren't strictly necessary, but some combination of them might be
> better than just the base patch, but splitting them out makes the
> individual changes easier to review.

Sorry for late reply. So I gave it a try but, apparently, our build
environment has changed quite significantly since the last time I
looked into it.

I don't see that many aa_get/put_buffer() anymore. apparmor buffer
functions are mostly called form the exec path:

	security_bprm_creds_for_exec()
	 apparmor_bprm_creds_for_exec()
	  make_vfsuid()
	   aa_get_buffer()

As for vfs_statx()->...->apparmor_inode_getattr()->aa_path_perm(),
that path is bpf_lsm_inode_getsecid() now.

Re: apparmor: global buffers spin lock may get contended

[John Johansen]

On 10/29/22 18:30, Hillf Danton wrote:
> On 28 Oct 2022 02:34:07 -0700 John Johansen <john.johansen@canonical.com>
>> On 7/13/21 06:19, Sergey Senozhatsky wrote:
>>> Hi,
>>>
>>> We've notices that apparmor has switched from using per-CPU buffer pool
>>> and per-CPU spin_lock to a global spin_lock in df323337e507a0009d3db1ea.
>>>
>>> This seems to be causing some contention on our build machines (with
>>> quite a bit of cores). Because that global spin lock is a part of the
>>> stat() sys call (and perhaps some other)
>>>
>>> E.g.
>>>
>>> -    9.29%     0.00%  clang++          [kernel.vmlinux]
>>>      - 9.28% entry_SYSCALL_64_after_hwframe
>>>         - 8.98% do_syscall_64
>>>            - 7.43% __do_sys_newlstat
>>>               - 7.43% vfs_statx
>>>                  - 7.18% security_inode_getattr
>>>                     - 7.15% apparmor_inode_getattr
>>>                        - aa_path_perm
>>>                           - 3.53% aa_get_buffer
>>>                              - 3.47% _raw_spin_lock
>>>                                   3.44% native_queued_spin_lock_slowpath
>>>                           - 3.49% aa_put_buffer.part.0
>>>                              - 3.45% _raw_spin_lock
>>>                                   3.43% native_queued_spin_lock_slowpath
>>>
>>> Can we fix this contention?
>>
>> sorry for the delay on this. Below is a proposed patch that I have been testing
>> to deal with this issue.
>>
>>
>>   From d026988196fdbda7234fb87bc3e4aea22edcbaf9 Mon Sep 17 00:00:00 2001
>> From: John Johansen <john.johansen@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.
>>
>> Signed-off-by: John Johansen <john.johansen@canonical.com>
>> ---
>>    security/apparmor/lsm.c | 74 ++++++++++++++++++++++++++++++++++++++---
>>    1 file changed, 69 insertions(+), 5 deletions(-)
>>
>> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
>> index 25114735bc11..0ab70171bdb6 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 +1704,32 @@ 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) {
>> +		put_cpu_ptr(&aa_local_buffers);
>> +		if (spin_trylock(&aa_buffers_lock)) {
>> +			list_add(&aa_buf->list, &aa_global_buffers);
>> +			buffer_count++;
> 
> Given !hold and trylock, right time to drain the perpcu cache?
> 

yes hold is a count of how long (or in this case a count of how many
times) to allocate from the local from the percpu cache before trying
to return to the global buffer pool. When the time/count hits zero
its time to try and return it.

If we succeed the try lock then we succeeded taking the global buffer
pool lock without contention and we can add the buffer back in.

As for the other cases

hold == 0 and fail to grab the lock
- contention is recorded and we add the buffer back to the percpu cache

hold > 0
- decrease hold and add back to the percpu cache

Since we never try and grab the spinlock if hold > 0, the lock variations
do not need to be considered.

>> +			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);
>> +	}
>> +
>> +	/* cache in percpu list */
>> +	list_add(&aa_buf->list, &cache->head);
>> +	put_cpu_ptr(&aa_local_buffers);
>>    }
>>    
>>    /*
>> @@ -1716,6 +1771,15 @@ 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