ftrace: Create a slight optimization on searching the ftrace_hash

This is a micro-optimization, but as it has to deal with a fast path of the
function tracer, these optimizations can be noticed.

The ftrace_lookup_ip() returns true if the given ip is found in the hash. If
it's not found or the hash is NULL, it returns false. But there's some cases
that a NULL hash is a true, and the ftrace_hash_empty() is tested before
calling ftrace_lookup_ip() in those cases. But as ftrace_lookup_ip() tests
that first, that adds a few extra unneeded instructions in those cases.

A new static "always_inlined" function is created that does not perform the
hash empty test. This most only be used by callers that do the check first
anyway, as an empty or NULL hash could cause a crash if a lookup is
performed on it.

Also add kernel doc for the ftrace_lookup_ip() main function.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -1194,16 +1194,14 @@ ftrace_hash_key(struct ftrace_hash *hash, unsigned long ip)
 	return 0;
 }
 
-struct ftrace_func_entry *
-ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip)
+/* Only use this function if ftrace_hash_empty() has already been tested */
+static __always_inline struct ftrace_func_entry *
+__ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip)
 {
 	unsigned long key;
 	struct ftrace_func_entry *entry;
 	struct hlist_head *hhd;
 
-	if (ftrace_hash_empty(hash))
-		return NULL;
-
 	key = ftrace_hash_key(hash, ip);
 	hhd = &hash->buckets[key];
 
@@ -1214,6 +1212,25 @@ ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip)
 	return NULL;
 }
 
+/**
+ * ftrace_lookup_ip - Test to see if an ip exists in an ftrace_hash
+ * @hash: The hash to look at
+ * @ip: The instruction pointer to test
+ *
+ * Search a given @hash to see if a given instruction pointer (@ip)
+ * exists in it.
+ *
+ * Returns the entry that holds the @ip if found. NULL otherwise.
+ */
+struct ftrace_func_entry *
+ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip)
+{
+	if (ftrace_hash_empty(hash))
+		return NULL;
+
+	return __ftrace_lookup_ip(hash, ip);
+}
+
 static void __add_hash_entry(struct ftrace_hash *hash,
 			     struct ftrace_func_entry *entry)
 {
@@ -1463,9 +1480,9 @@ static bool hash_contains_ip(unsigned long ip,
 	 * notrace hash is considered not in the notrace hash.
 	 */
 	return (ftrace_hash_empty(hash->filter_hash) ||
-		ftrace_lookup_ip(hash->filter_hash, ip)) &&
+		__ftrace_lookup_ip(hash->filter_hash, ip)) &&
 		(ftrace_hash_empty(hash->notrace_hash) ||
-		 !ftrace_lookup_ip(hash->notrace_hash, ip));
+		 !__ftrace_lookup_ip(hash->notrace_hash, ip));
 }
 
 /*
@@ -2877,7 +2894,7 @@ ops_references_rec(struct ftrace_ops *ops, struct dyn_ftrace *rec)
 
 	/* The function must be in the filter */
 	if (!ftrace_hash_empty(ops->func_hash->filter_hash) &&
-	    !ftrace_lookup_ip(ops->func_hash->filter_hash, rec->ip))
+	    !__ftrace_lookup_ip(ops->func_hash->filter_hash, rec->ip))
 		return 0;
 
 	/* If in notrace hash, we ignore it too */