uapi/linux/prctl: provide macro definitions for the PR_SCHED_CORE type argument

Commit 7ac592a ("sched: prctl() core-scheduling interface")
made use of enum pid_type in prctl's arg4; this type and the associated
enumeration definitions are not exposed to userspace.  Christian
has suggested to provide additional macro definitions that convey
the meaning of the type argument more in alignment with its actual
usage, and this patch does exactly that.

Link: https://lore.kernel.org/r/20210825170613.GA3884@asgard.redhat.com
Suggested-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Complements: 7ac592a ("sched: prctl() core-scheduling interface")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Author: esyr-rh

Documentation/admin-guide/hw-vuln/core-scheduling.rst

@@ -61,8 +61,9 @@ arg3:
     ``pid`` of the task for which the operation applies.
 
 arg4:
-    ``pid_type`` for which the operation applies. It is of type ``enum pid_type``.
-    For example, if arg4 is ``PIDTYPE_TGID``, then the operation of this command
+    ``pid_type`` for which the operation applies. It is one of
+    ``PR_SCHED_CORE_SCOPE_``-prefixed macro constants.  For example, if arg4
+    is ``PR_SCHED_CORE_SCOPE_THREAD_GROUP``, then the operation of this command
     will be performed for all tasks in the task group of ``pid``.
 
 arg5:

include/uapi/linux/prctl.h

@@ -268,5 +268,8 @@ struct prctl_mm_map {
 # define PR_SCHED_CORE_SHARE_TO		2 /* push core_sched cookie to pid */
 # define PR_SCHED_CORE_SHARE_FROM	3 /* pull core_sched cookie to pid */
 # define PR_SCHED_CORE_MAX		4
+# define PR_SCHED_CORE_SCOPE_THREAD		0
+# define PR_SCHED_CORE_SCOPE_THREAD_GROUP	1
+# define PR_SCHED_CORE_SCOPE_PROCESS_GROUP	2
 
 #endif /* _LINUX_PRCTL_H */

kernel/sched/core_sched.c

@@ -134,6 +134,10 @@ int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
 	if (!static_branch_likely(&sched_smt_present))
 		return -ENODEV;
 
+	BUILD_BUG_ON(PR_SCHED_CORE_SCOPE_THREAD != PIDTYPE_PID);
+	BUILD_BUG_ON(PR_SCHED_CORE_SCOPE_THREAD_GROUP != PIDTYPE_TGID);
+	BUILD_BUG_ON(PR_SCHED_CORE_SCOPE_PROCESS_GROUP != PIDTYPE_PGID);
+
 	if (type > PIDTYPE_PGID || cmd >= PR_SCHED_CORE_MAX || pid < 0 ||
 	    (cmd != PR_SCHED_CORE_GET && uaddr))
 		return -EINVAL;