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Subject[PATCH v2] sched/cputime: make scale_stime() more precise
People report that utime and stime from /proc/<pid>/stat become very
wrong when the numbers are big enough, especially if you watch these
counters incrementally.

Say, if the monitored process runs 100 days 50/50 in user/kernel mode
it looks as if it runs 20 minutes entirely in kernel mode, then 20
minutes in user mode. See the test-case which tries to demonstrate this
behaviour:

https://lore.kernel.org/lkml/20200124154215.GA14714@redhat.com/

The new implementation does the additional div64_u64_rem() but according
to my naive measurements it is faster on x86_64, much faster if rtime/etc
are big enough. See

https://lore.kernel.org/lkml/20200123130541.GA30620@redhat.com/

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/sched/cputime.c | 65 +++++++++++++++++++++++++-------------------------
1 file changed, 32 insertions(+), 33 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index d43318a..ae1ea09 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -528,42 +528,41 @@ void account_idle_ticks(unsigned long ticks)
*/
static u64 scale_stime(u64 stime, u64 rtime, u64 total)
{
- u64 scaled;
+ u64 res = 0, div, rem;
+ int shift;

- for (;;) {
- /* Make sure "rtime" is the bigger of stime/rtime */
- if (stime > rtime)
- swap(rtime, stime);
-
- /* 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;
+ /* can stime * rtime overflow ? */
+ if (ilog2(stime) + ilog2(rtime) > 62) {
+ /*
+ * (rtime * stime) / total is equal to
+ *
+ * (rtime / total) * stime +
+ * (rtime % total) * stime / total
+ *
+ * if nothing overflows. Can the 1st multiplication
+ * overflow? Yes, but we do not care: this can only
+ * happen if the end result can't fit in u64 anyway.
+ *
+ * So the code below does
+ *
+ * res = (rtime / total) * stime;
+ * rtime = rtime % total;
+ */
+ div = div64_u64_rem(rtime, total, &rem);
+ res = div * stime;
+ rtime = rem;
+
+ shift = ilog2(stime) + ilog2(rtime) - 62;
+ if (shift > 0) {
+ /* drop precision */
+ rtime >>= shift;
+ total >>= shift;
+ if (!total)
+ return res;
+ }
}

- /*
- * 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 scaled;
+ return res + div64_u64(stime * rtime, total);
}

/*
--
2.5.0

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