Re: [patch 2.6.16-mm2 8/9] sched throttle tree extract - maximize timeslice accounting

[Mike Galbraith <efault@gmx.de>]

This patch maximizes time slice accounting.  A task which receives too
much CPU time due to missing the timer interrupt will have the excess
deducted from it's next slice.

Signed-off-by: Mike Galbraith <efault@gmx.de>

--- linux-2.6.16-mm2/kernel/sched.c-7.implement_throttle	2006-03-24 09:36:08.000000000 +0100
+++ linux-2.6.16-mm2/kernel/sched.c	2006-03-24 09:40:33.000000000 +0100
@@ -2966,13 +2966,28 @@ static void refresh_timeslice(task_t *p)
 	unsigned int slice = last_slice(p);
 	unsigned int slice_avg, cpu, idle;
 	long run_time = -1 * p->slice_time_ns;
+	long slice_time_ns = task_timeslice_ns(p);
 	int w = MAX_BONUS, delta, bonus;
 
 	/*
-	 * Update time_slice.
+	 * Update time_slice.  Account for unused fragment,
+	 * or excess time received due to missed tick.
 	 */
-	p->slice_time_ns = task_timeslice_ns(p);
-	p->time_slice = task_timeslice(p);
+	p->slice_time_ns += slice_time_ns;
+	/*
+	 * Not common, but this does happen on SMP systems.
+	 * Timeslice theft of this magnitude has never been
+	 * observed in the wild, so assume that this is BS,
+	 * and give the poor task it's full slice.  Theory:
+	 * mostly idle task migrates between CPUs numerous
+	 * times during it's slice, timestamp rounding leads
+	 * to wildly inaccurate calculation.  Rounding has
+	 * maximum effect on those who stretch their slice,
+	 * but is also fairly meaningless, so ignore it.
+	 */
+	if (unlikely(p->slice_time_ns < NS_TICK))
+		p->slice_time_ns = slice_time_ns;
+	p->time_slice = NS_TO_JIFFIES(p->slice_time_ns);
 	set_last_slice(p, p->time_slice);
 
 	/*