cgroup: Wrap subsystem selection macro

Before we are able to define all subsystem ids at compile time we need
a more fine grained control what gets defined when we include
cgroup_subsys.h. For example we define the enums for the subsystems or
to declare for struct cgroup_subsys (builtin subsystem) by including
cgroup_subsys.h and defining SUBSYS accordingly.

Currently, the decision if a subsys is used is defined inside the
header by testing if CONFIG_*=y is true. By moving this test outside
of cgroup_subsys.h we are able to control it on the include level.

This is done by introducing IS_SUBSYS_ENABLED which then is defined
according the task, e.g. is CONFIG_*=y or CONFIG_*=m.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: netdev@vger.kernel.org
Cc: cgroups@vger.kernel.org

include/linux/cgroup.h

@@ -46,10 +46,12 @@ extern const struct file_operations proc_cgroup_operations;
 
 /* Define the enumeration of all builtin cgroup subsystems */
 #define SUBSYS(_x) _x ## _subsys_id,
+#define IS_SUBSYS_ENABLED(option) IS_BUILTIN(option)
 enum cgroup_subsys_id {
 #include <linux/cgroup_subsys.h>
 	__CGROUP_TEMPORARY_PLACEHOLDER
 };
+#undef IS_SUBSYS_ENABLED
 #undef SUBSYS
 /*
  * This define indicates the maximum number of subsystems that can be loaded
@@ -528,7 +530,9 @@ struct cgroup_subsys {
 };
 
 #define SUBSYS(_x) extern struct cgroup_subsys _x ## _subsys;
+#define IS_SUBSYS_ENABLED(option) IS_BUILTIN(option)
 #include <linux/cgroup_subsys.h>
+#undef IS_SUBSYS_ENABLED
 #undef SUBSYS
 
 static inline struct cgroup_subsys_state *cgroup_subsys_state(

include/linux/cgroup_subsys.h

@@ -7,73 +7,73 @@
 
 /* */
 
-#ifdef CONFIG_CPUSETS
+#if IS_SUBSYS_ENABLED(CONFIG_CPUSETS)
 SUBSYS(cpuset)
 #endif
 
 /* */
 
-#ifdef CONFIG_CGROUP_DEBUG
+#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_DEBUG)
 SUBSYS(debug)
 #endif
 
 /* */
 
-#ifdef CONFIG_CGROUP_SCHED
+#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_SCHED)
 SUBSYS(cpu_cgroup)
 #endif
 
 /* */
 
-#ifdef CONFIG_CGROUP_CPUACCT
+#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_CPUACCT)
 SUBSYS(cpuacct)
 #endif
 
 /* */
 
-#ifdef CONFIG_MEMCG
+#if IS_SUBSYS_ENABLED(CONFIG_MEMCG)
 SUBSYS(mem_cgroup)
 #endif
 
 /* */
 
-#ifdef CONFIG_CGROUP_DEVICE
+#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_DEVICE)
 SUBSYS(devices)
 #endif
 
 /* */
 
-#ifdef CONFIG_CGROUP_FREEZER
+#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_FREEZER)
 SUBSYS(freezer)
 #endif
 
 /* */
 
-#ifdef CONFIG_NET_CLS_CGROUP
+#if IS_SUBSYS_ENABLED(CONFIG_NET_CLS_CGROUP)
 SUBSYS(net_cls)
 #endif
 
 /* */
 
-#ifdef CONFIG_BLK_CGROUP
+#if IS_SUBSYS_ENABLED(CONFIG_BLK_CGROUP)
 SUBSYS(blkio)
 #endif
 
 /* */
 
-#ifdef CONFIG_CGROUP_PERF
+#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_PERF)
 SUBSYS(perf)
 #endif
 
 /* */
 
-#ifdef CONFIG_NETPRIO_CGROUP
+#if IS_SUBSYS_ENABLED(CONFIG_NETPRIO_CGROUP)
 SUBSYS(net_prio)
 #endif
 
 /* */
 
-#ifdef CONFIG_CGROUP_HUGETLB
+#if IS_SUBSYS_ENABLED(CONFIG_CGROUP_HUGETLB)
 SUBSYS(hugetlb)
 #endif
 

kernel/cgroup.c

@@ -93,6 +93,7 @@ static DEFINE_MUTEX(cgroup_root_mutex);
  * cgroup_mutex.
  */
 #define SUBSYS(_x) &_x ## _subsys,
+#define IS_SUBSYS_ENABLED(option) IS_BUILTIN(option)
 static struct cgroup_subsys *subsys[CGROUP_SUBSYS_COUNT] = {
 #include <linux/cgroup_subsys.h>
 };