[systemd-devel] [PATCH 1/2] RFC: util: Avoid memory allocations for formatting paths

[Holger Freyther]

From: Holger Hans Peter Freyther <holger at moiji-mobile.com>

Avoid memory allocations to construct the path for files in the
procfs. The procfs paths are way shorter than the PATH_MAX so we
can use snprintf on a string located on the stack. This shows up
as a win on x86 using the benchmark program below.

$ make libsystemd-shared.la; gcc -O2 -Isrc/systemd/ -Isrc/ \
	-o simple-perf-test simple-perf-test.c \
	.libs/libsystemd-shared.a  -lrt

 #include "shared/util.h"
void test_once(void) {
	pid_t pid = getpid();
	char *tmp = NULL;

	get_process_comm(pid, &tmp);
	free(tmp);
	tmp = NULL;
	get_process_cmdline(pid, 0, 1, &tmp);
	free(tmp);
	is_kernel_thread(pid);
	tmp = NULL;
	get_process_exe(pid, &tmp);
	free(tmp);
}

int main(int argc, char **argv)
{
	int i;
	for (i = 0; i < 50000; ++i)
		test_once();
}
---
 src/shared/util.c |   48 +++++++++++++++++++-----------------------------
 1 file changed, 19 insertions(+), 29 deletions(-)

diff --git a/src/shared/util.c b/src/shared/util.c
index b516b9b..ce8f866 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -567,6 +567,10 @@ char *truncate_nl(char *s) {
         return s;
 }
 
+#define format_procfs_path(buffer, path, pid) \
+        snprintf(buffer, sizeof(buffer) - 1, "/proc/%lu/%s", (unsigned long) pid, path); \
+        char_array_0(buffer);
+
 int get_process_comm(pid_t pid, char **name) {
         int r;
 
@@ -575,12 +579,9 @@ int get_process_comm(pid_t pid, char **name) {
         if (pid == 0)
                 r = read_one_line_file("/proc/self/comm", name);
         else {
-                char *p;
-                if (asprintf(&p, "/proc/%lu/comm", (unsigned long) pid) < 0)
-                        return -ENOMEM;
-
-                r = read_one_line_file(p, name);
-                free(p);
+                char path[PATH_MAX];
+                format_procfs_path(path, "comm", pid);
+                r = read_one_line_file(path, name);
         }
 
         return r;
@@ -596,12 +597,9 @@ int get_process_cmdline(pid_t pid, size_t max_length, bool comm_fallback, char *
         if (pid == 0)
                 f = fopen("/proc/self/cmdline", "re");
         else {
-                char *p;
-                if (asprintf(&p, "/proc/%lu/cmdline", (unsigned long) pid) < 0)
-                        return -ENOMEM;
-
-                f = fopen(p, "re");
-                free(p);
+                char path[PATH_MAX];
+                format_procfs_path(path, "cmdline", pid);
+                f = fopen(path, "re");
         }
 
         if (!f)
@@ -688,7 +686,7 @@ int get_process_cmdline(pid_t pid, size_t max_length, bool comm_fallback, char *
 }
 
 int is_kernel_thread(pid_t pid) {
-        char *p;
+        char path[PATH_MAX];
         size_t count;
         char c;
         bool eof;
@@ -697,11 +695,8 @@ int is_kernel_thread(pid_t pid) {
         if (pid == 0)
                 return 0;
 
-        if (asprintf(&p, "/proc/%lu/cmdline", (unsigned long) pid) < 0)
-                return -ENOMEM;
-
-        f = fopen(p, "re");
-        free(p);
+        format_procfs_path(path, "cmdline", pid);
+        f = fopen(path, "re");
 
         if (!f)
                 return -errno;
@@ -726,12 +721,9 @@ int get_process_exe(pid_t pid, char **name) {
         if (pid == 0)
                 r = readlink_malloc("/proc/self/exe", name);
         else {
-                char *p;
-                if (asprintf(&p, "/proc/%lu/exe", (unsigned long) pid) < 0)
-                        return -ENOMEM;
-
-                r = readlink_malloc(p, name);
-                free(p);
+                char path[PATH_MAX];
+                format_procfs_path(path, "exe", pid);
+                r = readlink_malloc(path, name);
         }
 
         return r;
@@ -739,7 +731,7 @@ int get_process_exe(pid_t pid, char **name) {
 
 static int get_process_id(pid_t pid, const char *field, uid_t *uid) {
         _cleanup_fclose_ FILE *f = NULL;
-        _cleanup_free_ char *p = NULL;
+        char path[PATH_MAX];
         char line[LINE_MAX];
 
         assert(field);
@@ -748,10 +740,8 @@ static int get_process_id(pid_t pid, const char *field, uid_t *uid) {
         if (pid == 0)
                 return getuid();
 
-        if (asprintf(&p, "/proc/%lu/status", (unsigned long) pid) < 0)
-                return -ENOMEM;
-
-        f = fopen(p, "re");
+        format_procfs_path(path, "status", pid);
+        f = fopen(path, "re");
         if (!f)
                 return -errno;
 
-- 
1.7.10.4