[tip: sched/core] sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime

[tip-bot2 for Zheng Zucheng]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     77baa5bafcbe1b2a15ef9c37232c21279c95481c
Gitweb:        https://git.kernel.org/tip/77baa5bafcbe1b2a15ef9c37232c21279c95481c
Author:        Zheng Zucheng <zhengzucheng@xxxxxxxxxx>
AuthorDate:    Fri, 26 Jul 2024 02:32:35 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 29 Jul 2024 12:22:32 +02:00

sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime

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
---
 kernel/sched/cputime.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index a5e0029..0bed0fa 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:
 	/*