From: Zheng Zucheng <zhengzucheng@xxxxxxxxxx>
commit 77baa5bafcbe1b2a15ef9c37232c21279c95481c upstream.
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
Fixes: 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()")
Signed-off-by: Zheng Zucheng <zhengzucheng@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20240726023235.217771-1-zhengzucheng@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/cputime.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -579,6 +579,12 @@ void cputime_adjust(struct task_cputime
}
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:
/*
Patches currently in stable-queue which might be from zhengzucheng@xxxxxxxxxx are
queue-5.10/sched-cputime-fix-mul_u64_u64_div_u64-precision-for-cputime.patch
