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: /*
