Read cpu.stat regardless if controller enabled.

The unified hierarchy provides the cpu.stat file for every cgroup,
regardless if the CPU controller is enabled (in fact, setting the
systemd property CPUAccounting=True does not enable this controller
because of this fact). It provides the usage_usec, user_usec,
and system_usec by default. Instead of reading the stat for each
enabled controller (CPU and memory), just attempt to read them
each time the Stat() function is called.

Attempting to read the memory.stat file even if memory accounting
is not enabled seems insignificant (some other files always have
a read attempt, such as memory.current), and eliminates finding and
looping over enabled controllers.

Resolves: containerd#347

Signed-off-by: Jackson McKay  <jackjaymckay@gmail.com>

cgroup2/manager.go

@@ -554,78 +554,61 @@ func (c *Manager) MoveTo(destination *Manager) error {
 }
 
 func (c *Manager) Stat() (*stats.Metrics, error) {
-	controllers, err := c.Controllers()
-	if err != nil {
-		return nil, err
-	}
-	// Sizing this avoids an allocation to increase the map at runtime;
-	// currently the default bucket size is 8 and we put 40+ elements
-	// in it so we'd always end up allocating.
-	out := make(map[string]uint64, 50)
-	for _, controller := range controllers {
-		switch controller {
-		case "cpu", "memory":
-			if err := readKVStatsFile(c.path, controller+".stat", out); err != nil {
-				if os.IsNotExist(err) {
-					continue
-				}
-				return nil, err
-			}
-		}
-	}
-	memoryEvents := make(map[string]uint64)
-	if err := readKVStatsFile(c.path, "memory.events", memoryEvents); err != nil {
+	var metrics stats.Metrics
+
+	cpuStat := make(map[string]uint64)
+	if err := readKVStatsFile(c.path, "cpu.stat", cpuStat); err != nil {
 		if !os.IsNotExist(err) {
 			return nil, err
 		}
 	}
-
-	var metrics stats.Metrics
-	metrics.Pids = &stats.PidsStat{
-		Current: getStatFileContentUint64(filepath.Join(c.path, "pids.current")),
-		Limit:   getStatFileContentUint64(filepath.Join(c.path, "pids.max")),
-	}
 	metrics.CPU = &stats.CPUStat{
-		UsageUsec:     out["usage_usec"],
-		UserUsec:      out["user_usec"],
-		SystemUsec:    out["system_usec"],
-		NrPeriods:     out["nr_periods"],
-		NrThrottled:   out["nr_throttled"],
-		ThrottledUsec: out["throttled_usec"],
+		UsageUsec:     cpuStat["usage_usec"],
+		UserUsec:      cpuStat["user_usec"],
+		SystemUsec:    cpuStat["system_usec"],
+		NrPeriods:     cpuStat["nr_periods"],
+		NrThrottled:   cpuStat["nr_throttled"],
+		ThrottledUsec: cpuStat["throttled_usec"],
 		PSI:           getStatPSIFromFile(filepath.Join(c.path, "cpu.pressure")),
 	}
+	memoryStat := make(map[string]uint64, 40)
+	if err := readKVStatsFile(c.path, "memory.stat", memoryStat); err != nil {
+		if !os.IsNotExist(err) {
+			return nil, err
+		}
+	}
 	metrics.Memory = &stats.MemoryStat{
-		Anon:                  out["anon"],
-		File:                  out["file"],
-		KernelStack:           out["kernel_stack"],
-		Slab:                  out["slab"],
-		Sock:                  out["sock"],
-		Shmem:                 out["shmem"],
-		FileMapped:            out["file_mapped"],
-		FileDirty:             out["file_dirty"],
-		FileWriteback:         out["file_writeback"],
-		AnonThp:               out["anon_thp"],
-		InactiveAnon:          out["inactive_anon"],
-		ActiveAnon:            out["active_anon"],
-		InactiveFile:          out["inactive_file"],
-		ActiveFile:            out["active_file"],
-		Unevictable:           out["unevictable"],
-		SlabReclaimable:       out["slab_reclaimable"],
-		SlabUnreclaimable:     out["slab_unreclaimable"],
-		Pgfault:               out["pgfault"],
-		Pgmajfault:            out["pgmajfault"],
-		WorkingsetRefault:     out["workingset_refault"],
-		WorkingsetActivate:    out["workingset_activate"],
-		WorkingsetNodereclaim: out["workingset_nodereclaim"],
-		Pgrefill:              out["pgrefill"],
-		Pgscan:                out["pgscan"],
-		Pgsteal:               out["pgsteal"],
-		Pgactivate:            out["pgactivate"],
-		Pgdeactivate:          out["pgdeactivate"],
-		Pglazyfree:            out["pglazyfree"],
-		Pglazyfreed:           out["pglazyfreed"],
-		ThpFaultAlloc:         out["thp_fault_alloc"],
-		ThpCollapseAlloc:      out["thp_collapse_alloc"],
+		Anon:                  memoryStat["anon"],
+		File:                  memoryStat["file"],
+		KernelStack:           memoryStat["kernel_stack"],
+		Slab:                  memoryStat["slab"],
+		Sock:                  memoryStat["sock"],
+		Shmem:                 memoryStat["shmem"],
+		FileMapped:            memoryStat["file_mapped"],
+		FileDirty:             memoryStat["file_dirty"],
+		FileWriteback:         memoryStat["file_writeback"],
+		AnonThp:               memoryStat["anon_thp"],
+		InactiveAnon:          memoryStat["inactive_anon"],
+		ActiveAnon:            memoryStat["active_anon"],
+		InactiveFile:          memoryStat["inactive_file"],
+		ActiveFile:            memoryStat["active_file"],
+		Unevictable:           memoryStat["unevictable"],
+		SlabReclaimable:       memoryStat["slab_reclaimable"],
+		SlabUnreclaimable:     memoryStat["slab_unreclaimable"],
+		Pgfault:               memoryStat["pgfault"],
+		Pgmajfault:            memoryStat["pgmajfault"],
+		WorkingsetRefault:     memoryStat["workingset_refault"],
+		WorkingsetActivate:    memoryStat["workingset_activate"],
+		WorkingsetNodereclaim: memoryStat["workingset_nodereclaim"],
+		Pgrefill:              memoryStat["pgrefill"],
+		Pgscan:                memoryStat["pgscan"],
+		Pgsteal:               memoryStat["pgsteal"],
+		Pgactivate:            memoryStat["pgactivate"],
+		Pgdeactivate:          memoryStat["pgdeactivate"],
+		Pglazyfree:            memoryStat["pglazyfree"],
+		Pglazyfreed:           memoryStat["pglazyfreed"],
+		ThpFaultAlloc:         memoryStat["thp_fault_alloc"],
+		ThpCollapseAlloc:      memoryStat["thp_collapse_alloc"],
 		Usage:                 getStatFileContentUint64(filepath.Join(c.path, "memory.current")),
 		UsageLimit:            getStatFileContentUint64(filepath.Join(c.path, "memory.max")),
 		MaxUsage:              getStatFileContentUint64(filepath.Join(c.path, "memory.peak")),
@@ -634,6 +617,13 @@ func (c *Manager) Stat() (*stats.Metrics, error) {
 		SwapMaxUsage:          getStatFileContentUint64(filepath.Join(c.path, "memory.swap.peak")),
 		PSI:                   getStatPSIFromFile(filepath.Join(c.path, "memory.pressure")),
 	}
+
+	memoryEvents := make(map[string]uint64)
+	if err := readKVStatsFile(c.path, "memory.stat", memoryStat); err != nil {
+		if !os.IsNotExist(err) {
+			return nil, err
+		}
+	}
 	if len(memoryEvents) > 0 {
 		metrics.MemoryEvents = &stats.MemoryEvents{
 			Low:     memoryEvents["low"],