[PATCH v2 -next] sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime

[Zheng Zucheng]

In extreme test scenarios:
the 14th field utime in /proc/xx/stat is greater than sum_exec_runtime,
utime = 18446744073709518790 ns, rtime = 135989749728000 ns

In cputime_adjust() process, stime is greater than rtime due to
mul_u64_u64_div_u64() precision problem.
before call mul_u64_u64_div_u64(),
stime = 175136586720000, rtime = 135989749728000, utime = 1416780000.
after call mul_u64_u64_div_u64(),
stime = 135989949653530

unsigned reversion occurs because rtime is less than stime.
utime = rtime - stime = 135989749728000 - 135989949653530
		      = -199925530
		      = (u64)18446744073709518790

Trigger condition:
1). User task run in kernel mode most of time
2). ARM64 architecture
3). TICK_CPU_ACCOUNTING=y
    CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is not set

Fix mul_u64_u64_div_u64() conversion precision by reset stime to rtime

v2:
 - Add comment
 - Update trigger condition

Fixes: 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Zheng Zucheng <zhengzucheng@xxxxxxxxxx>
---
 kernel/sched/cputime.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index aa48b2ec879d..4feef0d4e449 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -582,6 +582,12 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
 	}
 
 	stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
+	/*
+	 * Because mul_u64_u64_div_u64() can approximate on some
+	 * achitectures; enforce the constraint that: a*b/(b+c) <= a.
+	 */
+	if (unlikely(stime > rtime))
+		stime = rtime;
 
 update:
 	/*
-- 
2.34.1