[PATCH] Fix task_hot() balancing

This fixes the integer underflow in task_hot() noticed by Kenneth W Chen
and makes use of p->last_ran to separate load-balancing timestamps (used
by task_hot()) from interactivity timestamps.  (which two interfered)

compiled, booted on x86 SMP.

Confirmed by Kenneth Chen <kenneth.w.chen@intel.com> to fix the db
transaction processing workload that showed the balancing problem.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/sched.h

@@ -449,7 +449,7 @@ struct task_struct {
 
 	unsigned long sleep_avg;
 	long interactive_credit;
-	unsigned long long timestamp;
+	unsigned long long timestamp, last_ran;
 	int activated;
 
 	unsigned long policy;

kernel/sched.c

@@ -180,7 +180,8 @@ static unsigned int task_timeslice(task_t *p)
 	else
 		return SCALE_PRIO(DEF_TIMESLICE, p->static_prio);
 }
-#define task_hot(p, now, sd) ((now) - (p)->timestamp < (sd)->cache_hot_time)
+#define task_hot(p, now, sd) ((long long) ((now) - (p)->last_ran)	\
+				< (long long) (sd)->cache_hot_time)
 
 enum idle_type
 {
@@ -2764,7 +2765,7 @@ asmlinkage void __sched schedule(void)
 		if (!(HIGH_CREDIT(prev) || LOW_CREDIT(prev)))
 			prev->interactive_credit--;
 	}
-	prev->timestamp = now;
+	prev->timestamp = prev->last_ran = now;
 
 	sched_info_switch(prev, next);
 	if (likely(prev != next)) {