Merge pull request #10507 from cdown/cpu_acct

cgroup v2: Don't require CPU controller for CPU accounting in 4.15+

src/basic/cgroup-util.c

@@ -12,6 +12,7 @@
 #include <sys/stat.h>
 #include <sys/statfs.h>
 #include <sys/types.h>
+#include <sys/utsname.h>
 #include <sys/xattr.h>
 #include <unistd.h>
 
@@ -2822,3 +2823,54 @@ static const char *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 (str_verscmp(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;
+}

src/basic/cgroup-util.h

@@ -69,6 +69,9 @@ 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_t) -1)
 #define CGROUP_WEIGHT_MIN UINT64_C(1)