sched: Fix granularity of task_u/stime()

Originally task_s/utime() were designed to return clock_t but
later changed to return cputime_t by following commit:

  commit efe567fc8281661524ffa75477a7c4ca9b466c63
  Author: Christian Borntraeger <borntraeger@de.ibm.com>
  Date:   Thu Aug 23 15:18:02 2007 +0200

It only changed the type of return value, but not the
implementation. As the result the granularity of task_s/utime()
is still that of clock_t, not that of cputime_t.

So using task_s/utime() in __exit_signal() makes values
accumulated to the signal struct to be rounded and coarse
grained.

This patch removes casts to clock_t in task_u/stime(), to keep
granularity of cputime_t over the calculation.

v2:
  Use div_u64() to avoid error "undefined reference to `__udivdi3`"
  on some 32bit systems.

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: xiyou.wangcong@gmail.com
Cc: Spencer Candland <spencer@bluehost.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
LKML-Reference: <4AFB9029.9000208@jp.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 13 insertions, 9 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 43e61fa04dc7..ab9a034c4a17 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5156,41 +5156,45 @@ cputime_t task_stime(struct task_struct *p)
         return p->stime;
 }
 #else
+
+#ifndef nsecs_to_cputime
+# define nsecs_to_cputime(__nsecs) \
+        msecs_to_cputime(div_u64((__nsecs), NSEC_PER_MSEC))
+#endif
+
 cputime_t task_utime(struct task_struct *p)
 {
-        clock_t utime = cputime_to_clock_t(p->utime),
-                total = utime + cputime_to_clock_t(p->stime);
+        cputime_t utime = p->utime, total = utime + p->stime;
         u64 temp;
 
         /*
          * Use CFS's precise accounting:
          */
-        temp = (u64)nsec_to_clock_t(p->se.sum_exec_runtime);
+        temp = (u64)nsecs_to_cputime(p->se.sum_exec_runtime);
 
         if (total) {
                 temp *= utime;
                 do_div(temp, total);
         }
-        utime = (clock_t)temp;
+        utime = (cputime_t)temp;
 
-        p->prev_utime = max(p->prev_utime, clock_t_to_cputime(utime));
+        p->prev_utime = max(p->prev_utime, utime);
         return p->prev_utime;
 }
 
 cputime_t task_stime(struct task_struct *p)
 {
-        clock_t stime;
+        cputime_t stime;
 
         /*
          * Use CFS's precise accounting. (we subtract utime from
          * the total, to make sure the total observed by userspace
          * grows monotonically - apps rely on that):
          */
-        stime = nsec_to_clock_t(p->se.sum_exec_runtime) -
-                        cputime_to_clock_t(task_utime(p));
+        stime = nsecs_to_cputime(p->se.sum_exec_runtime) - task_utime(p);
 
         if (stime >= 0)
-                p->prev_stime = max(p->prev_stime, clock_t_to_cputime(stime));
+                p->prev_stime = max(p->prev_stime, stime);
 
         return p->prev_stime;
 }