linux: read load average from /proc/loadavg

It was reported that uv_loadavg() reports the wrong values inside an
lxc container.

Libuv calls sysinfo(2) but that isn't intercepted by lxc. /proc/loadavg
however is because /proc is a FUSE fs inside the container.

This commit makes libuv try /proc/loadavg first and fall back to
sysinfo(2) in case /proc isn't mounted.

This commit is very similar to commit 3a1be72 ("linux: read free/total
memory from /proc/meminfo") from April 2019.

Fixes: nodejs/node#33791

CMakeLists.txt

@@ -192,8 +192,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Android")
        src/unix/pthread-fixes.c
        src/unix/random-getentropy.c
        src/unix/random-getrandom.c
-       src/unix/random-sysctl-linux.c
-       src/unix/sysinfo-loadavg.c)
+       src/unix/random-sysctl-linux.c)
 endif()
 
 if(APPLE OR CMAKE_SYSTEM_NAME MATCHES "Android|Linux|OS390")
@@ -237,8 +236,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
        src/unix/linux-syscalls.c
        src/unix/procfs-exepath.c
        src/unix/random-getrandom.c
-       src/unix/random-sysctl-linux.c
-       src/unix/sysinfo-loadavg.c)
+       src/unix/random-sysctl-linux.c)
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "NetBSD")

Makefile.am

@@ -386,8 +386,7 @@ libuv_la_SOURCES += src/unix/android-ifaddrs.c \
                     src/unix/procfs-exepath.c \
                     src/unix/pthread-fixes.c \
                     src/unix/random-getrandom.c \
-                    src/unix/random-sysctl-linux.c \
-                    src/unix/sysinfo-loadavg.c
+                    src/unix/random-sysctl-linux.c
 endif
 
 if CYGWIN
@@ -468,8 +467,7 @@ libuv_la_SOURCES += src/unix/linux-core.c \
                     src/unix/procfs-exepath.c \
                     src/unix/proctitle.c \
                     src/unix/random-getrandom.c \
-                    src/unix/random-sysctl-linux.c \
-                    src/unix/sysinfo-loadavg.c
+                    src/unix/random-sysctl-linux.c
 test_run_tests_LDFLAGS += -lutil
 endif
 

src/unix/linux-core.c

@@ -1084,3 +1084,20 @@ uint64_t uv_get_constrained_memory(void) {
    */
   return uv__read_cgroups_uint64("memory", "memory.limit_in_bytes");
 }
+
+
+void uv_loadavg(double avg[3]) {
+  struct sysinfo info;
+  char buf[128];  /* Large enough to hold all of /proc/loadavg. */
+
+  if (0 == uv__slurp("/proc/loadavg", buf, sizeof(buf)))
+    if (3 == sscanf(buf, "%lf %lf %lf", &avg[0], &avg[1], &avg[2]))
+      return;
+
+  if (sysinfo(&info) < 0)
+    return;
+
+  avg[0] = (double) info.loads[0] / 65536.0;
+  avg[1] = (double) info.loads[1] / 65536.0;
+  avg[2] = (double) info.loads[2] / 65536.0;
+}