[PATCH v7 05/11] kallsyms: Improve the performance of kallsyms_lookup_name()

[Zhen Lei]

Currently, to search for a symbol, we need to expand the symbols in
'kallsyms_names' one by one, and then use the expanded string for
comparison. This process can be optimized.

And now scripts/kallsyms no longer compresses the symbol types, each
symbol type always occupies one byte. So we can first compress the
searched symbol and then make a quick comparison based on the compressed
length and content. In this way, for entries with mismatched lengths,
there is no need to expand and compare strings. And for those matching
lengths, there's no need to expand the symbol. This saves a lot of time.
According to my test results, the average performance of
kallsyms_lookup_name() can be improved by 20 to 30 times.

The pseudo code of the test case is as follows:
static int stat_find_name(...)
{
	start = sched_clock();
	(void)kallsyms_lookup_name(name);
	end = sched_clock();
	//Update min, max, cnt, sum
}

/*
 * Traverse all symbols in sequence and collect statistics on the time
 * taken by kallsyms_lookup_name() to lookup each symbol.
 */
kallsyms_on_each_symbol(stat_find_name, NULL);

The test results are as follows (twice):
After : min=5250, max=  726560, avg= 302132
After : min=5320, max=  726850, avg= 301978
Before: min=170,  max=15949190, avg=7553906
Before: min=160,  max=15877280, avg=7517784

The average time consumed is only 4.01% and the maximum time consumed is
only 4.57% of the time consumed before optimization.

Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
---
 kernel/kallsyms.c | 50 +++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 46 insertions(+), 4 deletions(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index f1fe404af184047..7f3987cc975be3b 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -107,7 +107,7 @@ static unsigned char *find_token(unsigned char *str, int len,
 	return NULL;
 }
 
-static int __maybe_unused kallsyms_compress_symbol_name(const char *name, char *buf, size_t size)
+static int kallsyms_compress_symbol_name(const char *name, char *buf, size_t size)
 {
 	int i, j, n, len;
 	unsigned char *p1, *p2;
@@ -267,23 +267,65 @@ static bool cleanup_symbol_name(char *s)
 	return false;
 }
 
+static int kallsyms_lookup_compressed_name(unsigned char *namebuf, int namelen,
+					   unsigned long *addr)
+{
+	unsigned int i, off;
+	unsigned int len, x;
+	const unsigned char *name;
+
+	for (i = 0, off = 0; namelen && i < kallsyms_num_syms; i++) {
+		/*
+		 * For each entry in kallsyms_names[], the storage format is:
+		 *  ----------------------------
+		 * | len(1-2) | type(1) | name(x) |
+		 *  ----------------------------
+		 *
+		 * Number of bytes in parentheses, and: len = 1 + x
+		 */
+		len = kallsyms_names[off];
+		off++;
+		if (len & 0x80) {
+			len = (len & 0x7f) | (kallsyms_names[off] << 7);
+			off++;
+		}
+		name = &kallsyms_names[off + 1];
+		off += len;
+
+		x = len - 1;
+		if (x != namelen)
+			continue;
+
+		if (!memcmp(name, namebuf, namelen)) {
+			*addr = kallsyms_sym_address(i);
+			return 0;
+		}
+	}
+
+	return -ENOENT;
+}
+
 /* Lookup the address for this symbol. Returns 0 if not found. */
 unsigned long kallsyms_lookup_name(const char *name)
 {
 	char namebuf[KSYM_NAME_LEN];
 	unsigned long i;
 	unsigned int off;
+	unsigned long addr;
+	int ret, len;
 
 	/* Skip the search for empty string. */
 	if (!*name)
 		return 0;
 
+	len = kallsyms_compress_symbol_name(name, namebuf, ARRAY_SIZE(namebuf));
+	ret = kallsyms_lookup_compressed_name(namebuf, len, &addr);
+	if (!ret)
+		return addr;
+
 	for (i = 0, off = 0; i < kallsyms_num_syms; i++) {
 		off = kallsyms_expand_symbol(off, namebuf, ARRAY_SIZE(namebuf));
 
-		if (strcmp(namebuf, name) == 0)
-			return kallsyms_sym_address(i);
-
 		if (cleanup_symbol_name(namebuf) && strcmp(namebuf, name) == 0)
 			return kallsyms_sym_address(i);
 	}
-- 
2.25.1