[GIT PULL] scheduler fixes

Ingo Molnar
2013-05-02T09:06:27Z

Linus,

Please pull the latest sched-urgent-for-linus git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched-urgent-for-linus

HEAD: f3002134158092178be81339ec5a22ff80e6c308 Revert "math64: New div64_u64_rem helper"

This fixes the cputime scaling overflow problems for good without having
bad 32-bit overhead, and gets rid of the div64_u64_rem() helper as well.

Ingo

------------------>
Stanislaw Gruszka (4):
sched: Avoid cputime scaling overflow
sched: Do not account bogus utime
sched: Avoid prev->stime underflow
Revert "math64: New div64_u64_rem helper"

include/linux/math64.h | 19 +-----------
kernel/sched/cputime.c | 80 ++++++++++++++++++++++++++++++++------------------
lib/div64.c | 19 ++++--------
3 files changed, 58 insertions(+), 60 deletions(-)

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 931a619..b8ba855 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -30,15 +30,6 @@ static inline s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder)
}

/**
- * div64_u64_rem - unsigned 64bit divide with 64bit divisor
- */
-static inline u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
-{
- *remainder = dividend % divisor;
- return dividend / divisor;
-}
-
-/**
* div64_u64 - unsigned 64bit divide with 64bit divisor
*/
static inline u64 div64_u64(u64 dividend, u64 divisor)
@@ -70,16 +61,8 @@ static inline u64 div_u64_rem(u64 dividend, u32 divisor, u32 *remainder)
extern s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder);
#endif

-#ifndef div64_u64_rem
-extern u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder);
-#endif
-
#ifndef div64_u64
-static inline u64 div64_u64(u64 dividend, u64 divisor)
-{
- u64 remainder;
- return div64_u64_rem(dividend, divisor, &remainder);
-}
+extern u64 div64_u64(u64 dividend, u64 divisor);
#endif

#ifndef div64_s64
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 33508dc..337a367 100644
--- 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;
}
diff --git a/lib/div64.c b/lib/div64.c
index 3af5728..a163b6c 100644
--- a/lib/div64.c
+++ b/lib/div64.c
@@ -79,10 +79,9 @@ EXPORT_SYMBOL(div_s64_rem);
#endif

/**
- * div64_u64_rem - unsigned 64bit divide with 64bit divisor and 64bit remainder
+ * div64_u64 - unsigned 64bit divide with 64bit divisor
* @dividend: 64bit dividend
* @divisor: 64bit divisor
- * @remainder: 64bit remainder
*
* This implementation is a modified version of the algorithm proposed
* by the book 'Hacker's Delight'. The original source and full proof
@@ -90,33 +89,27 @@ EXPORT_SYMBOL(div_s64_rem);
*
* 'http://www.hackersdelight.org/HDcode/newCode/divDouble.c.txt'
*/
-#ifndef div64_u64_rem
-u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
+#ifndef div64_u64
+u64 div64_u64(u64 dividend, u64 divisor)
{
u32 high = divisor >> 32;
u64 quot;

if (high == 0) {
- u32 rem32;
- quot = div_u64_rem(dividend, divisor, &rem32);
- *remainder = rem32;
+ quot = div_u64(dividend, divisor);
} else {
int n = 1 + fls(high);
quot = div_u64(dividend >> n, divisor >> n);

if (quot != 0)
quot--;
-
- *remainder = dividend - quot * divisor;
- if (*remainder >= divisor) {
+ if ((dividend - quot * divisor) >= divisor)
quot++;
- *remainder -= divisor;
- }
}

return quot;
}
-EXPORT_SYMBOL(div64_u64_rem);
+EXPORT_SYMBOL(div64_u64);
#endif

/**