[PATCH] sched: fix bug in __migrate_task()

Problem:

In the function __migrate_task(), deactivate_task() followed by
activate_task() is used to move the task from one run queue to
another.  This has two undesirable effects:

1. The task's priority is recalculated. (Nowhere else in the
scheduler code is the priority recalculated for a change of CPU.)

2. The task's time stamp is set to the current time.  At the very least,
this makes the adjustment of the time stamp before the call to
deactivate_task() redundant but I believe the problem is more serious
as the time stamp now holds the time of the queue change instead of
the time at which the task was woken.  In addition, unless dest_rq is
the same queue as "current" is on the time stamp could be inaccurate
due to inter CPU drift.

Solution:

Replace the call to activate_task() with one to __activate_task().
Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -4877,7 +4877,7 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 		p->timestamp = p->timestamp - rq_src->timestamp_last_tick
 				+ rq_dest->timestamp_last_tick;
 		deactivate_task(p, rq_src);
-		activate_task(p, rq_dest, 0);
+		__activate_task(p, rq_dest);
 		if (TASK_PREEMPTS_CURR(p, rq_dest))
 			resched_task(rq_dest->curr);
 	}