Adding cgroups path to the Spec.

Signed-off-by: Vishnu Kannan <vishnuk@google.com>

Author: vishh

runtime-config-linux.md

@@ -130,6 +130,57 @@ Next parameters can be specified:
 Also known as cgroups, they are used to restrict resource usage for a container and handle
 device access.  cgroups provide controls to restrict cpu, memory, IO, and network for
 the container. For more information, see the [kernel cgroups documentation](https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt).
+The path to the cgroups can to be specified in the Spec via `cgroupsPath`.
+`cgroupsPath` is expected to be relative to the cgroups mount point.
+If not specified, cgroups will be created under '/' with the base name of the bundle.
+The Spec does not support [split hierarchy](https://www.kernel.org/doc/Documentation/cgroups/unified-hierarchy.txt).  
+The cgroups will be created if they don't exist.
+
+```json
+   "cgroupsPath": "/myRuntime/myContainer"
+```
+
+`cgroupsPath` can be used to either control the cgroups hierarchy for containers or to run a new process in an existing container.
+
+Optionally, cgroups limits can be specified via `resources`. 
+
+```json
+    "resources": {
+        "disableOOMKiller": false,
+        "memory": {
+            "limit": 0,
+            "reservation": 0,
+            "swap": 0,
+            "kernel": 0,
+            "swappiness": -1
+        },
+        "cpu": {
+            "shares": 0,
+            "quota": 0,
+            "period": 0,
+            "realtimeRuntime": 0,
+            "realtimePeriod": 0,
+            "cpus": "",
+            "mems": ""
+        },
+        "blockIO": {
+            "blkioWeight": 0,
+            "blkioWeightDevice": "",
+            "blkioThrottleReadBpsDevice": "",
+            "blkioThrottleWriteBpsDevice": "",
+            "blkioThrottleReadIopsDevice": "",
+            "blkioThrottleWriteIopsDevice": ""
+        },
+        "hugepageLimits": null,
+        "network": {
+            "classId": "",
+            "priorities": null
+        }
+    }
+```
+
+Do not specify `resources` unless limits have to be updated. 
+For example, to run a new process in an existing container without updating limits, `resources` need not be specified.
 
 ## Sysctl
 

runtime_config_linux.go

@@ -20,7 +20,11 @@ type LinuxRuntime struct {
 	Sysctl map[string]string `json:"sysctl"`
 	// Resources contain cgroup information for handling resource constraints
 	// for the container
-	Resources Resources `json:"resources"`
+	Resources *Resources `json:"resources"`
+	// CgroupsPath specifies the path to cgroups that are created and/or joined by the container.
+	// The path is expected to be relative to the cgroups mountpoint.
+	// If resources are specified, the cgroups at CgroupsPath will be updated based on resources.
+	CgroupsPath string `json:"cgroupsPath"`
 	// Namespaces contains the namespaces that are created and/or joined by the container
 	Namespaces []Namespace `json:"namespaces"`
 	// Devices are a list of device nodes that are created and enabled for the container