[PATCH] sched: improve efficiency of sched_fork()

Problem:
  sched_fork() has always called scheduler_tick() in some (unlikely)
  circumstances in order to update the current task in light of those
  circumstances.  It has always been the case that the work done by
  scheduler_tick() was more than was required to handle the problem in
  hand but no harm was done except for the waste of a few CPU cycles.

  However, the splitting of scheduler_tick() into two procedures in
  2.6.20-rc1 enables the wasted cycles to be saved as the new procedure
  task_running_tick() does all the work that is required to rectify the
  problem being handled.

Solution:
  Replace the call to scheduler_tick() in sched_fork() with a call to
  task_running_tick().
Signed-off-by: NPeter Williams <pwil3058@bigpond.com.au>
Acked-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -1567,6 +1567,7 @@ int fastcall wake_up_state(struct task_struct *p, unsigned int state)
 	return try_to_wake_up(p, state, 0);
 }
 
+static void task_running_tick(struct rq *rq, struct task_struct *p);
 /*
  * Perform scheduler related setup for a newly forked process p.
  * p is forked by current.
@@ -1627,7 +1628,7 @@ void fastcall sched_fork(struct task_struct *p, int clone_flags)
 		 * runqueue lock is not a problem.
 		 */
 		current->time_slice = 1;
-		scheduler_tick();
+		task_running_tick(cpu_rq(cpu), current);
 	}
 	local_irq_enable();
 	put_cpu();