core: always enable CPU accounting

Our baseline is v5.4 and cgroup v2 is enforced now,
which means CPU accounting is cheap everywhere without
requiring any controller, hence just remove the directive.

src/basic/cgroup-util.c

@@ -2181,57 +2181,6 @@ static const char *const cgroup_controller_table[_CGROUP_CONTROLLER_MAX] = {
 
 DEFINE_STRING_TABLE_LOOKUP(cgroup_controller, CGroupController);
 
-CGroupMask get_cpu_accounting_mask(void) {
-        static CGroupMask needed_mask = (CGroupMask) -1;
-
-        /* On kernel ≥4.15 with unified hierarchy, cpu.stat's usage_usec is
-         * provided externally from the CPU controller, which means we don't
-         * need to enable the CPU controller just to get metrics. This is good,
-         * because enabling the CPU controller comes at a minor performance
-         * hit, especially when it's propagated deep into large hierarchies.
-         * There's also no separate CPU accounting controller available within
-         * a unified hierarchy.
-         *
-         * This combination of factors results in the desired cgroup mask to
-         * enable for CPU accounting varying as follows:
-         *
-         *                   ╔═════════════════════╤═════════════════════╗
-         *                   ║     Linux ≥4.15     │     Linux <4.15     ║
-         *   ╔═══════════════╬═════════════════════╪═════════════════════╣
-         *   ║ Unified       ║ nothing             │ CGROUP_MASK_CPU     ║
-         *   ╟───────────────╫─────────────────────┼─────────────────────╢
-         *   ║ Hybrid/Legacy ║ CGROUP_MASK_CPUACCT │ CGROUP_MASK_CPUACCT ║
-         *   ╚═══════════════╩═════════════════════╧═════════════════════╝
-         *
-         * We check kernel version here instead of manually checking whether
-         * cpu.stat is present for every cgroup, as that check in itself would
-         * already be fairly expensive.
-         *
-         * Kernels where this patch has been backported will therefore have the
-         * CPU controller enabled unnecessarily. This is more expensive than
-         * necessary, but harmless. ☺️
-         */
-
-        if (needed_mask == (CGroupMask) -1) {
-                if (cg_all_unified()) {
-                        struct utsname u;
-                        assert_se(uname(&u) >= 0);
-
-                        if (strverscmp_improved(u.release, "4.15") < 0)
-                                needed_mask = CGROUP_MASK_CPU;
-                        else
-                                needed_mask = 0;
-                } else
-                        needed_mask = CGROUP_MASK_CPUACCT;
-        }
-
-        return needed_mask;
-}
-
-bool cpu_accounting_is_cheap(void) {
-        return get_cpu_accounting_mask() == 0;
-}
-
 static const char* const managed_oom_mode_table[_MANAGED_OOM_MODE_MAX] = {
         [MANAGED_OOM_AUTO] = "auto",
         [MANAGED_OOM_KILL] = "kill",

src/basic/cgroup-util.h

@@ -86,9 +86,6 @@ static inline CGroupMask CGROUP_MASK_EXTEND_JOINED(CGroupMask mask) {
         return mask;
 }
 
-CGroupMask get_cpu_accounting_mask(void);
-bool cpu_accounting_is_cheap(void);
-
 /* Special values for all weight knobs on unified hierarchy */
 #define CGROUP_WEIGHT_INVALID UINT64_MAX
 #define CGROUP_WEIGHT_IDLE UINT64_C(0)