core: support percentage specifications on TasksMax=

This adds support for a TasksMax=40% syntax for specifying values relative to
the system's configured maximum number of processes. This is useful in order to
neatly subdivide the available room for tasks within containers.

src/basic/util.c

@@ -832,6 +832,61 @@ uint64_t physical_memory_scale(uint64_t v, uint64_t max) {
         return r;
 }
 
+uint64_t system_tasks_max(void) {
+
+#if SIZEOF_PID_T == 4
+#define TASKS_MAX ((uint64_t) (INT32_MAX-1))
+#elif SIZEOF_PID_T == 2
+#define TASKS_MAX ((uint64_t) (INT16_MAX-1))
+#else
+#error "Unknown pid_t size"
+#endif
+
+        _cleanup_free_ char *value = NULL, *root = NULL;
+        uint64_t a = TASKS_MAX, b = TASKS_MAX;
+
+        /* Determine the maximum number of tasks that may run on this system. We check three sources to determine this
+         * limit:
+         *
+         * a) the maximum value for the pid_t type
+         * b) the cgroups pids_max attribute for the system
+         * c) the kernel's configure maximum PID value
+         *
+         * And then pick the smallest of the three */
+
+        if (read_one_line_file("/proc/sys/kernel/pid_max", &value) >= 0)
+                (void) safe_atou64(value, &a);
+
+        if (cg_get_root_path(&root) >= 0) {
+                value = mfree(value);
+
+                if (cg_get_attribute("pids", root, "pids.max", &value) >= 0)
+                        (void) safe_atou64(value, &b);
+        }
+
+        return MIN3(TASKS_MAX,
+                    a <= 0 ? TASKS_MAX : a,
+                    b <= 0 ? TASKS_MAX : b);
+}
+
+uint64_t system_tasks_max_scale(uint64_t v, uint64_t max) {
+        uint64_t t, m;
+
+        assert(max > 0);
+
+        /* Multiply the system's task value by the fraction v/max. Hence, if max==100 this calculates percentages
+         * relative to the system's maximum number of tasks. Returns UINT64_MAX on overflow. */
+
+        t = system_tasks_max();
+        assert(t > 0);
+
+        m = t * v;
+        if (m / t != v) /* overflow? */
+                return UINT64_MAX;
+
+        return m / max;
+}
+
 int update_reboot_parameter_and_warn(const char *param) {
         int r;