[PATCH] sched: fix timeslice calculations for HZ=1000.

The main benefit is that with the default HZ=1000 nice +19 tasks now get 5
msecs of timeslices, so the ratio of CPU use is linear.  (nice 0 task gets
20 times more CPU time than a nice 19 task.  Prior this change the ratio
was 1:10)

another effect is that nice 0 tasks now get a round 100 msecs of timeslices
(as intended), instead of 102 msecs.

here's a table of old/new timeslice values, for HZ=1000 and 100:

                      HZ=1000         (   HZ=100   )
                    old    new        ( old    new )

        nice -20:   200    200        ( 200    200 )
        nice -19:   195    195        ( 190    190 )
        ...
        nice 0:     102    100        ( 100    100 )
        nice 1:      97     95        (  90     90 )
        nice 2:      92     90        (  90     90 )
        ...
        nice 17:     19     15        (  10     10 )
        nice 18:     14     10        (  10     10 )
        nice 19:     10      5        (  10     10 )

i've tested the patch on x86.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -79,11 +79,11 @@
 /*
  * These are the 'tuning knobs' of the scheduler:
  *
- * Minimum timeslice is 10 msecs, default timeslice is 100 msecs,
- * maximum timeslice is 200 msecs. Timeslices get refilled after
- * they expire.
+ * Minimum timeslice is 5 msecs (or 1 jiffy, whichever is larger),
+ * default timeslice is 100 msecs, maximum timeslice is 200 msecs.
+ * Timeslices get refilled after they expire.
  */
-#define MIN_TIMESLICE		( 10 * HZ / 1000)
+#define MIN_TIMESLICE		max(5 * HZ / 1000, 1)
 #define MAX_TIMESLICE		(200 * HZ / 1000)
 #define ON_RUNQUEUE_WEIGHT	 30
 #define CHILD_PENALTY		 95
@@ -171,9 +171,9 @@
  * task_timeslice() is the interface that is used by the scheduler.
  */
 
-#define BASE_TIMESLICE(p) (MIN_TIMESLICE + \
-		((MAX_TIMESLICE - MIN_TIMESLICE) * \
-			(MAX_PRIO-1 - (p)->static_prio) / (MAX_USER_PRIO-1)))
+#define BASE_TIMESLICE(p) \
+	max(MAX_TIMESLICE * (MAX_PRIO - (p)->static_prio) / (MAX_USER_PRIO), \
+		MIN_TIMESLICE)
 
 static unsigned int task_timeslice(task_t *p)
 {