[PATCH] 2.6.6. runqueue lock for RT priority tasks

[Dimitri Sivanich]

Hi,

In the scheduler_tick code, the runqueue lock is currently taken before
checking whether a task is an RT task or not.  It would be more efficient to
take the lock only in cases where the task is not RT or the task policy is
SCHED_RR with no timeslice left.

I welcome your comments on the 2.6.6 patch presented below.

Thanks,

Dimitri Sivanich <sivanich@xxxxxxx>


Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c	2004-05-10 15:29:33.000000000 -0500
+++ linux/kernel/sched.c	2004-05-10 15:29:33.000000000 -0500
@@ -1521,7 +1521,6 @@
 		set_tsk_need_resched(p);
 		goto out;
 	}
-	spin_lock(&rq->lock);
 	/*
 	 * The task was running during this tick - update the
 	 * time slice counter. Note: we do not update a thread's
@@ -1535,6 +1534,7 @@
 		 * FIFO tasks have no timeslices.
 		 */
 		if ((p->policy == SCHED_RR) && !--p->time_slice) {
+			spin_lock(&rq->lock);
 			p->time_slice = task_timeslice(p);
 			p->first_time_slice = 0;
 			set_tsk_need_resched(p);
@@ -1542,9 +1542,11 @@
 			/* put it at the end of the queue: */
 			dequeue_task(p, rq->active);
 			enqueue_task(p, rq->active);
+			goto out_unlock;
 		}
-		goto out_unlock;
+		goto out;
 	}
+	spin_lock(&rq->lock);
 	if (!--p->time_slice) {
 		dequeue_task(p, rq->active);
 		set_tsk_need_resched(p);
-
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