Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel...

[~andy/linux] / kernel / sched / cputime.c

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index ea32f02bf2c37674820c4b2dd94b38979b2895aa..cc2dc3eea8a3a55d7c86cc9896bfe9d235c18cab 100644 (file)
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -506,34 +506,47 @@ void account_idle_ticks(unsigned long ticks)
 }
 
 /*
- * Perform (stime * rtime) / total with reduced chances
- * of multiplication overflows by using smaller factors
- * like quotient and remainders of divisions between
- * rtime and total.
+ * Perform (stime * rtime) / total, but avoid multiplication overflow by
+ * loosing precision when the numbers are big.
  */
 static cputime_t scale_stime(u64 stime, u64 rtime, u64 total)
 {
-       u64 rem, res, scaled;
+       u64 scaled;
 
-       if (rtime >= total) {
-               /*
-                * Scale up to rtime / total then add
-                * the remainder scaled to stime / total.
-                */
-               res = div64_u64_rem(rtime, total, &rem);
-               scaled = stime * res;
-               scaled += div64_u64(stime * rem, total);
-       } else {
-               /*
-                * Same in reverse: scale down to total / rtime
-                * then substract that result scaled to
-                * to the remaining part.
-                */
-               res = div64_u64_rem(total, rtime, &rem);
-               scaled = div64_u64(stime, res);
-               scaled -= div64_u64(scaled * rem, total);
+       for (;;) {
+               /* Make sure "rtime" is the bigger of stime/rtime */
+               if (stime > rtime) {
+                       u64 tmp = rtime; rtime = stime; stime = tmp;
+               }
+
+               /* Make sure 'total' fits in 32 bits */
+               if (total >> 32)
+                       goto drop_precision;
+
+               /* Does rtime (and thus stime) fit in 32 bits? */
+               if (!(rtime >> 32))
+                       break;
+
+               /* Can we just balance rtime/stime rather than dropping bits? */
+               if (stime >> 31)
+                       goto drop_precision;
+
+               /* We can grow stime and shrink rtime and try to make them both fit */
+               stime <<= 1;
+               rtime >>= 1;
+               continue;
+
+drop_precision:
+               /* We drop from rtime, it has more bits than stime */
+               rtime >>= 1;
+               total >>= 1;
        }
 
+       /*
+        * Make sure gcc understands that this is a 32x32->64 multiply,
+        * followed by a 64/32->64 divide.
+        */
+       scaled = div_u64((u64) (u32) stime * (u64) (u32) rtime, (u32)total);
        return (__force cputime_t) scaled;
 }
 
@@ -545,7 +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;
@@ -568,13 +581,21 @@ static void cputime_adjust(struct task_cputime *curr,
         */
        rtime = nsecs_to_cputime(curr->sum_exec_runtime);
 
-       if (!rtime) {
-               stime = 0;
-       } else if (!total) {
-               stime = rtime;
-       } else {
+       /*
+        * 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 (total) {
                stime = scale_stime((__force u64)stime,
                                    (__force u64)rtime, (__force u64)total);
+               utime = rtime - stime;
+       } else {
+               stime = rtime;
+               utime = 0;
        }
 
        /*
@@ -583,8 +604,9 @@ 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;
        *st = prev->stime;
 }