cfq-iosched: make io priorities inherit CPU scheduling class as well as nice

We currently set all processes to the best-effort scheduling class,
regardless of what CPU scheduling class they belong to. Improve that
so that we correctly track idle and rt scheduling classes as well.

Signed-off-by: Jens Axboe <[e-mail]>

block/cfq-iosched.c

···
 		printk(KERN_ERR "cfq: bad prio %x\n", ioprio_class);
 	case IOPRIO_CLASS_NONE:
 		/*
-		 * no prio set, place us in the middle of the BE classes
+		 * no prio set, inherit CPU scheduling settings
 		 */
 		cfqq->ioprio = task_nice_ioprio(tsk);
-		cfqq->ioprio_class = IOPRIO_CLASS_BE;
+		cfqq->ioprio_class = task_nice_ioclass(tsk);
 		break;
 	case IOPRIO_CLASS_RT:
 		cfqq->ioprio = task_ioprio(ioc);

include/linux/ioprio.h

···
 }
 
 /*
+ * This is for the case where the task hasn't asked for a specific IO class.
+ * Check for idle and rt task process, and return appropriate IO class.
+ */
+static inline int task_nice_ioclass(struct task_struct *task)
+{
+	if (task->policy == SCHED_IDLE)
+		return IOPRIO_CLASS_IDLE;
+	else if (task->policy == SCHED_FIFO || task->policy == SCHED_RR)
+		return IOPRIO_CLASS_RT;
+	else
+		return IOPRIO_CLASS_BE;
+}
+
+/*
  * For inheritance, return the highest of the two given priorities
  */
 extern int ioprio_best(unsigned short aprio, unsigned short bprio);