prctl: adds PR_SET_TIMERSLACK_PID for setting timer slack of an arbitrary thread.

Second argument is similar to PR_SET_TIMERSLACK, if non-zero then the
slack is set to that value otherwise sets it to the default for the thread.

Takes PID of the thread as the third argument.

This allows power/performance management software to set timer slack for
other threads according to its policy for the thread (such as when the
thread is designated foreground vs. background activity)

Change-Id: I744d451ff4e60dae69f38f53948ff36c51c14a3f
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>

include/uapi/linux/prctl.h

@@ -197,4 +197,10 @@ struct prctl_mm_map {
 # define PR_CAP_AMBIENT_LOWER		3
 # define PR_CAP_AMBIENT_CLEAR_ALL	4
 
+/* Sets the timerslack for arbitrary threads
+ * arg2 slack value, 0 means "use default"
+ * arg3 pid of the thread whose timer slack needs to be set
+ */
+#define PR_SET_TIMERSLACK_PID 47
+
 #endif /* _LINUX_PRCTL_H */

kernel/sys.c

@@ -2076,6 +2076,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
 {
 	struct task_struct *me = current;
+	struct task_struct *tsk;
 	unsigned char comm[sizeof(me->comm)];
 	long error;
 
@@ -2199,6 +2200,23 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			else
 				return -EINVAL;
 			break;
+		case PR_SET_TIMERSLACK_PID:
+			rcu_read_lock();
+			tsk = find_task_by_pid_ns((pid_t)arg3, &init_pid_ns);
+			if (tsk == NULL) {
+				rcu_read_unlock();
+				return -EINVAL;
+			}
+			get_task_struct(tsk);
+			rcu_read_unlock();
+			if (arg2 <= 0)
+				tsk->timer_slack_ns =
+					tsk->default_timer_slack_ns;
+			else
+				tsk->timer_slack_ns = arg2;
+			put_task_struct(tsk);
+			error = 0;
+			break;
 		default:
 			return -EINVAL;
 		}