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

[zhengzucheng]

Sorry, I made a mistake here. CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is not set.

在 2024/7/25 22:05, Peter Zijlstra 写道:
> 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@xxxxxxxxxx>
> > ---
> >   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!
>
>
> .