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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 25, 2024 at 12:03:15PM +0000, Zheng Zucheng wrote:
> 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;

But but but... for CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y the code you're
patching is not compiled!