[PATCH v4 2/2] audit: Use the new {get,put}_fs_pwd_pool() APIs to get/put pwd references

[Waiman Long <longman@redhat.com>]

When the audit subsystem is enabled, it can do a lot of get_fs_pwd()
calls to get references to fs->pwd and then releasing those references
back with path_put() later. That may cause a lot of spinlock contention
on a single pwd's dentry lock because of the constant changes to the
reference count when there are many processes on the same working
directory actively doing open/close system calls. This can cause
noticeable performance regresssion when compared with the case where
the audit subsystem is turned off especially on systems with a lot of
CPUs which is becoming more common these days.

To avoid this kind of performance regression, use the new
get_fs_pwd_pool() and put_fs_pwd_pool() APIs to acquire and release a
fs->pwd reference. This should greatly reduce the number of path_get()
and path_put() calls that are needed.

After installing a test kernel with auditing enabled and counters
added to track the get_fs_pwd_pool() and put_fs_pwd_pool() calls on
a 2-socket 96-core test system and running a parallel kernel build,
the counter values for this particular test run were shown below.

  fs_get_path=307,903
  fs_get_pool=56,583,192
  fs_put_path=6,209
  fs_put_pool=56,885,147

Of the about 57M calls to get_fs_pwd_pool() and put_fs_pwd_pool(), the
majority of them are just updating the pwd_refs counters. Only less than
1% of those calls require an actual path_get() and path_put() calls. The
difference between fs_get_path and fs_put_path represents the extra pwd
references that were still stored in various active task->fs's when the
counter values were retrieved.

It can be seen that the number of path_get() and path_put() calls are
reduced by quite a lot.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
---
 kernel/auditsc.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index f6af6a8f68c4..26ba61eabfb0 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -931,6 +931,9 @@ static inline void audit_free_names(struct audit_context *context)
 {
 	struct audit_names *n, *next;
 
+	if (!context->name_count)
+		return;	/* audit_alloc_name() has not been called */
+
 	list_for_each_entry_safe(n, next, &context->names_list, list) {
 		list_del(&n->list);
 		if (n->name)
@@ -939,7 +942,7 @@ static inline void audit_free_names(struct audit_context *context)
 			kfree(n);
 	}
 	context->name_count = 0;
-	path_put(&context->pwd);
+	put_fs_pwd_pool(current->fs, &context->pwd);
 	context->pwd.dentry = NULL;
 	context->pwd.mnt = NULL;
 }
@@ -2165,7 +2168,7 @@ static struct audit_names *audit_alloc_name(struct audit_context *context,
 
 	context->name_count++;
 	if (!context->pwd.dentry)
-		get_fs_pwd(current->fs, &context->pwd);
+		get_fs_pwd_pool(current->fs, &context->pwd);
 	return aname;
 }
 
-- 
2.53.0