sched: keep utime/stime monotonic

sched: keep utime/stime monotonic

cpustats use utime/stime as a ratio against sum_exec_runtime, as a
consequence it can happen - when the ratio changes faster than time
accumulates - that either can be appear to go backwards.

Combined backport for 2.6.23 of the following patches from mainline:
commit 73a2bcb0
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
  sched: keep utime/stime monotonic

commit 9301899b
Author: Balbir Singh <balbir@linux.vnet.ibm.com>
  sched: fix /proc/<PID>/stat stime/utime monotonicity, part 2
Signed-off-by: Frans Pop <elendil@planet.nl>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/proc/array.c

@@ -351,7 +351,8 @@ static cputime_t task_utime(struct task_struct *p)
 	}
 	utime = (clock_t)temp;
 
-	return clock_t_to_cputime(utime);
+	p->prev_utime = max(p->prev_utime, clock_t_to_cputime(utime));
+	return p->prev_utime;
 }
 
 static cputime_t task_stime(struct task_struct *p)
@@ -366,7 +367,8 @@ static cputime_t task_stime(struct task_struct *p)
 	stime = nsec_to_clock_t(p->se.sum_exec_runtime) -
 			cputime_to_clock_t(task_utime(p));
 
-	return clock_t_to_cputime(stime);
+	p->prev_stime = max(p->prev_stime, clock_t_to_cputime(stime));
+	return p->prev_stime;
 }
 #endif
 

include/linux/sched.h

@@ -1022,6 +1022,7 @@ struct task_struct {
 
 	unsigned int rt_priority;
 	cputime_t utime, stime;
+	cputime_t prev_utime, prev_stime;
 	unsigned long nvcsw, nivcsw; /* context switch counts */
 	struct timespec start_time; 		/* monotonic time */
 	struct timespec real_start_time;	/* boot based time */

kernel/fork.c

@@ -1045,6 +1045,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 
 	p->utime = cputime_zero;
 	p->stime = cputime_zero;
+	p->prev_utime = cputime_zero;
+	p->prev_stime = cputime_zero;
 
 #ifdef CONFIG_TASK_XACCT
 	p->rchar = 0;		/* I/O counter: bytes read */