sched: Avoid prev->stime underflow

Dave Hansen reported strange utime/stime values on his system:
https://lkml.org/lkml/2013/4/4/435

This happens because prev->stime value is bigger than rtime
value. Root of the problem are non-monotonic rtime values (i.e.
current rtime is smaller than previous rtime) and that should be
debugged and fixed.

But since problem did not manifest itself before commit
62188451f0d63add7ad0cd2a1ae269d600c1663d "cputime: Avoid
multiplication overflow on utime scaling", it should be threated
as regression, which we can easily fixed on cputime_adjust()
function.

For now, let's apply this fix, but further work is needed to fix
root of the problem.

Reported-and-tested-by: Dave Hansen <dave@sr71.net>
Cc: <stable@vger.kernel.org> # 3.9+
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/1367314507-9728-3-git-send-email-sgruszka@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 1b7c2161f5cd..337a36745800 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -558,7 +558,7 @@ static void cputime_adjust(struct task_cputime *curr,
                            struct cputime *prev,
                            cputime_t *ut, cputime_t *st)
 {
-        cputime_t rtime, stime, total;
+        cputime_t rtime, stime, utime, total;
 
         if (vtime_accounting_enabled()) {
                 *ut = curr->utime;
@@ -589,13 +589,13 @@ static void cputime_adjust(struct task_cputime *curr,
         if (prev->stime + prev->utime >= rtime)
                 goto out;
 
-        if (!rtime) {
-                stime = 0;
-        } else if (!total) {
-                stime = rtime;
-        } else {
+        if (total) {
                 stime = scale_stime((__force u64)stime,
                                     (__force u64)rtime, (__force u64)total);
+                utime = rtime - stime;
+        } else {
+                stime = rtime;
+                utime = 0;
         }
 
         /*
@@ -604,7 +604,7 @@ static void cputime_adjust(struct task_cputime *curr,
          * Let's enforce monotonicity.
          */
         prev->stime = max(prev->stime, stime);
-        prev->utime = max(prev->utime, rtime - prev->stime);
+        prev->utime = max(prev->utime, utime);
 
 out:
         *ut = prev->utime;