linux: fix uv_available_parallelism using cgroup (#4278)

uv_available_parallelism does not handle container cpu limit
set by systems like Docker or Kubernetes. This patch fixes
this limitation by comparing the amount of available cpus
returned by syscall with the quota of cpus available defined
in the cgroup.

Fixes: https://github.com/libuv/libuv/issues/4146

src/unix/core.c

@@ -1869,6 +1869,8 @@ unsigned int uv_available_parallelism(void) {
 #ifdef __linux__
   cpu_set_t set;
   long rc;
+  double rc_with_cgroup;
+  uv__cpu_constraint c = {0, 0, 0.0};
 
   memset(&set, 0, sizeof(set));
 
@@ -1880,8 +1882,13 @@ unsigned int uv_available_parallelism(void) {
     rc = CPU_COUNT(&set);
   else
     rc = sysconf(_SC_NPROCESSORS_ONLN);
-
-  if (rc < 1)
+    
+  if (uv__get_constrained_cpu(&c) == 0 && c.period_length > 0) {
+    rc_with_cgroup = (double)c.quota_per_period / c.period_length * c.proportions;
+    if (rc_with_cgroup < rc)
+      rc = (long)rc_with_cgroup; /* Casting is safe since rc_with_cgroup < rc < LONG_MAX */
+  }
+  if (rc < 1) 
     rc = 1;
 
   return (unsigned) rc;

src/unix/internal.h

@@ -472,4 +472,14 @@ uv__fs_copy_file_range(int fd_in,
 #define UV__CPU_AFFINITY_SUPPORTED 0
 #endif
 
+#ifdef __linux__
+typedef struct {
+  long long quota_per_period;
+  long long period_length;
+  double proportions;
+} uv__cpu_constraint;
+
+int uv__get_constrained_cpu(uv__cpu_constraint* constraint);
+#endif
+
 #endif /* UV_UNIX_INTERNAL_H_ */

src/unix/linux.c

@@ -2278,6 +2278,136 @@ uint64_t uv_get_available_memory(void) {
 }
 
 
+static int uv__get_cgroupv2_constrained_cpu(const char* cgroup, 
+                                            uv__cpu_constraint* constraint) {
+  char path[256];
+  char buf[1024];
+  unsigned int weight;
+  int cgroup_size;
+  const char* cgroup_trimmed;
+  char quota_buf[16];
+
+  if (strncmp(cgroup, "0::/", 4) != 0)
+    return UV_EINVAL;
+   
+  /* Trim ending \n by replacing it with a 0 */
+  cgroup_trimmed = cgroup + sizeof("0::/") - 1;      /* Skip the prefix "0::/" */
+  cgroup_size = (int)strcspn(cgroup_trimmed, "\n");  /* Find the first slash */
+
+  /* Construct the path to the cpu.max file */
+  snprintf(path, sizeof(path), "/sys/fs/cgroup/%.*s/cpu.max", cgroup_size,
+           cgroup_trimmed);
+
+  /* Read cpu.max */
+  if (uv__slurp(path, buf, sizeof(buf)) < 0)
+    return UV_EIO;
+
+  if (sscanf(buf, "%15s %llu", quota_buf, &constraint->period_length) != 2)
+    return UV_EINVAL;
+
+  if (strncmp(quota_buf, "max", 3) == 0)
+    constraint->quota_per_period = LLONG_MAX;
+  else if (sscanf(quota_buf, "%lld", &constraint->quota_per_period) != 1)
+    return UV_EINVAL; // conversion failed
+
+  /* Construct the path to the cpu.weight file */
+  snprintf(path, sizeof(path), "/sys/fs/cgroup/%.*s/cpu.weight", cgroup_size,
+           cgroup_trimmed);
+
+  /* Read cpu.weight */
+  if (uv__slurp(path, buf, sizeof(buf)) < 0)
+    return UV_EIO;
+
+  if (sscanf(buf, "%u", &weight) != 1)
+    return UV_EINVAL;
+
+  constraint->proportions = (double)weight / 100.0;
+
+  return 0;
+}
+
+static char* uv__cgroup1_find_cpu_controller(const char* cgroup,
+                                             int* cgroup_size) {
+  /* Seek to the cpu controller line. */
+  char* cgroup_cpu = strstr(cgroup, ":cpu,");
+
+  if (cgroup_cpu != NULL) {
+    /* Skip the controller prefix to the start of the cgroup path. */
+    cgroup_cpu += sizeof(":cpu,") - 1;
+    /* Determine the length of the cgroup path, excluding the newline. */
+    *cgroup_size = (int)strcspn(cgroup_cpu, "\n");
+  }
+
+  return cgroup_cpu;
+}
+
+static int uv__get_cgroupv1_constrained_cpu(const char* cgroup, 
+                                            uv__cpu_constraint* constraint) {
+  char path[256];
+  char buf[1024];
+  unsigned int shares;
+  int cgroup_size;
+  char* cgroup_cpu;
+
+  cgroup_cpu = uv__cgroup1_find_cpu_controller(cgroup, &cgroup_size);
+
+  if (cgroup_cpu == NULL)
+    return UV_EIO;
+
+  /* Construct the path to the cpu.cfs_quota_us file */
+  snprintf(path, sizeof(path), "/sys/fs/cgroup/%.*s/cpu.cfs_quota_us",
+           cgroup_size, cgroup_cpu);
+
+  if (uv__slurp(path, buf, sizeof(buf)) < 0)
+    return UV_EIO;  
+    
+  if (sscanf(buf, "%lld", &constraint->quota_per_period) != 1)
+    return UV_EINVAL;
+
+  /* Construct the path to the cpu.cfs_period_us file */
+  snprintf(path, sizeof(path), "/sys/fs/cgroup/%.*s/cpu.cfs_period_us",
+           cgroup_size, cgroup_cpu);
+
+  /* Read cpu.cfs_period_us */
+  if (uv__slurp(path, buf, sizeof(buf)) < 0)
+    return UV_EIO;
+
+  if (sscanf(buf, "%lld", &constraint->period_length) != 1)
+    return UV_EINVAL;
+
+  /* Construct the path to the cpu.shares file */
+  snprintf(path, sizeof(path), "/sys/fs/cgroup/%.*s/cpu.shares", cgroup_size,
+           cgroup_cpu);
+
+  /* Read cpu.shares */
+  if (uv__slurp(path, buf, sizeof(buf)) < 0)
+    return UV_EIO;
+  
+  if (sscanf(buf, "%u", &shares) != 1)
+    return UV_EINVAL;
+
+  constraint->proportions = (double)shares / 1024.0;
+
+  return 0;
+}
+
+int uv__get_constrained_cpu(uv__cpu_constraint* constraint) {
+  char cgroup[1024];
+
+  /* Read the cgroup from /proc/self/cgroup */
+  if (uv__slurp("/proc/self/cgroup", cgroup, sizeof(cgroup)) < 0)
+    return UV_EIO;
+
+  /* Check if the system is using cgroup v2 by examining /proc/self/cgroup
+   * The entry for cgroup v2 is always in the format "0::$PATH"
+   * see https://docs.kernel.org/admin-guide/cgroup-v2.html */
+  if (strncmp(cgroup, "0::/", 4) == 0)
+    return uv__get_cgroupv2_constrained_cpu(cgroup, constraint);
+  else
+    return uv__get_cgroupv1_constrained_cpu(cgroup, constraint);
+}
+
+
 void uv_loadavg(double avg[3]) {
   struct sysinfo info;
   char buf[128];  /* Large enough to hold all of /proc/loadavg. */

test/test-platform-output.c

@@ -90,6 +90,45 @@ TEST_IMPL(platform_output) {
   ASSERT_GE(par, 1);
   printf("uv_available_parallelism: %u\n", par);
 
+#ifdef __linux__
+  FILE* file;
+  int cgroup_version = 0;
+  unsigned int cgroup_par = 0;
+  uint64_t quota, period;
+
+  // Attempt to parse cgroup v2 to deduce parallelism constraints
+  file = fopen("/sys/fs/cgroup/cpu.max", "r");
+  if (file) {
+    if (fscanf(file, "%lu %lu", &quota, &period) == 2 && quota > 0) {
+      cgroup_version = 2;
+      cgroup_par = (unsigned int)(quota / period);
+    }
+    fclose(file);
+  }
+
+  // If cgroup v2 wasn't present, try parsing cgroup v1
+  if (cgroup_version == 0) {
+    file = fopen("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_quota_us", "r");
+    if (file) {
+      if (fscanf(file, "%lu", &quota) == 1 && quota > 0) {
+        fclose(file);
+        file = fopen("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_period_us", "r");
+        if (file && fscanf(file, "%lu", &period) == 1) {
+          cgroup_version = 1;
+          cgroup_par = (unsigned int)(quota / period);
+        }
+      }
+      if (file) fclose(file);
+    }
+  }
+
+  // If we found cgroup parallelism constraints, assert and print them
+  if (cgroup_par > 0) {
+    ASSERT_GE(par, cgroup_par);
+    printf("cgroup v%d available parallelism: %u\n", cgroup_version, cgroup_par);
+  }
+#endif
+
   err = uv_cpu_info(&cpus, &count);
 #if defined(__CYGWIN__) || defined(__MSYS__)
   ASSERT_EQ(err, UV_ENOSYS);