Re: [PATCH (update 3)] timer: Run calc_load halfway through each round_jiffies second

[Simon Arlott <simon@fire.lp0.eu>]

Whenever jiffies is started at a multiple of 5*HZ or wraps, calc_load is 
run exactly on the second which is when tasks using round_jiffies will 
be scheduled to run. This has a bad effect on the load average, making 
it tend towards 1.00 if a task happens to run every time the load is 
being calculated.

This changes calc_load so that it updates load half a second after any 
tasks scheduled using round_jiffies.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
---
This version starts count off at 0, since LOAD_FREQ + HZ/2 is no better 
than LOAD_FREQ. The previous version is actually wrong, rounding count to 
a negative value will only happen when getting calc_load running at the
right time related to jiffies, it won't be negative running normally - 
even with NO_HZ.

 kernel/timer.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/timer.c b/kernel/timer.c
index 6663a87..4abead0 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1226,17 +1226,19 @@ EXPORT_SYMBOL(avenrun);
 static inline void calc_load(unsigned long ticks)
 {
 	unsigned long active_tasks; /* fixed-point */
-	static int count = LOAD_FREQ;
+	static int count = 0;
 
 	count -= ticks;
-	if (unlikely(count < 0)) {
+	if (unlikely(count <= 0)) {
 		active_tasks = count_active_tasks();
 		do {
 			CALC_LOAD(avenrun[0], EXP_1, active_tasks);
 			CALC_LOAD(avenrun[1], EXP_5, active_tasks);
 			CALC_LOAD(avenrun[2], EXP_15, active_tasks);
 			count += LOAD_FREQ;
-		} while (count < 0);
+		} while (count <= 0);
+
+		count = round_jiffies_relative(count + HZ/2) - HZ/2;
 	}
 }
 
-- 
1.5.0.1


-- 
Simon Arlott