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

Zheng Zucheng
2024-07-25T12:04:49Z

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 scenario:
1. User task run in kernel mode most of time.
2. The ARM64 architecture && CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y &&
TICK_CPU_ACCOUNTING=y

Fix mul_u64_u64_div_u64() conversion precision by reset stime to rtime

Fixes: 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()")
Signed-off-by: Zheng Zucheng <zhengzucheng@huawei.com>
---
kernel/sched/cputime.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index aa48b2ec879d..365c74e95537 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -582,6 +582,8 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
}

stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
+ if (unlikely(stime > rtime))
+ stime = rtime;

update:
/*
--
2.34.1