sched: Do not account bogus utime

commit 772c808a252594692972773f6ee41c289b8e0b2a upstream.

Due to rounding in scale_stime(), for big numbers, scaled stime
values will grow in chunks. Since rtime grow in jiffies and we
calculate utime like below:

	prev->stime = max(prev->stime, stime);
	prev->utime = max(prev->utime, rtime - prev->stime);

we could erroneously account stime values as utime. To prevent
that only update prev->{u,s}time values when they are smaller
than current rtime.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: rostedt@goodmis.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Hansen <dave@sr71.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1367314507-9728-2-git-send-email-sgruszka@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index a514a879e94667317e736087f2f349390ae5a4eb..910d1f5be442cd678ad0466661866bbc37bbf930 100644 (file)
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -591,6 +591,14 @@ static void cputime_adjust(struct task_cputime *curr,
         */
        rtime = nsecs_to_cputime(curr->sum_exec_runtime);
 
+       /*
+        * Update userspace visible utime/stime values only if actual execution
+        * time is bigger than already exported. Note that can happen, that we
+        * provided bigger values due to scaling inaccuracy on big numbers.
+        */
+       if (prev->stime + prev->utime >= rtime)
+               goto out;
+
        if (!rtime) {
                stime = 0;
        } else if (!total) {
@@ -608,6 +616,7 @@ static void cputime_adjust(struct task_cputime *curr,
        prev->stime = max(prev->stime, stime);
        prev->utime = max(prev->utime, rtime - prev->stime);
 
+out:
        *ut = prev->utime;
        *st = prev->stime;
 }