Re: [BUG nohz]: wrong user and system time accounting

Thomas Gleixner
2017-04-11T14:22:54Z

On Thu, 30 Mar 2017, Wanpeng Li wrote:
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index f3778e2b..f1ee393 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -676,18 +676,21 @@ void thread_group_cputime_adjusted(struct
> task_struct *p, u64 *ut, u64 *st)
> #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
> static u64 vtime_delta(struct task_struct *tsk)
> {
> - unsigned long now = READ_ONCE(jiffies);
> + u64 now = local_clock();
> + u64 delta;
> +
> + delta = now - tsk->vtime_snap;
>
> - if (time_before(now, (unsigned long)tsk->vtime_snap))
> + if (delta < TICK_NSEC)
> return 0;
>
> - return jiffies_to_nsecs(now - tsk->vtime_snap);
> + return jiffies_to_nsecs(delta / TICK_NSEC);

So you replaced a jiffies based approach with a jiffies based approach.

> }
>
> static u64 get_vtime_delta(struct task_struct *tsk)
> {
> - unsigned long now = READ_ONCE(jiffies);
> - u64 delta, other;
> + u64 delta = vtime_delta(tsk);
> + u64 other;
>
> /*
> * Unlike tick based timing, vtime based timing never has lost
> @@ -696,10 +699,9 @@ static u64 get_vtime_delta(struct task_struct *tsk)
> * elapsed time. Limit account_other_time to prevent rounding
> * errors from causing elapsed vtime to go negative.
> */
> - delta = jiffies_to_nsecs(now - tsk->vtime_snap);
> other = account_other_time(delta);
> WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);
> - tsk->vtime_snap = now;
> + tsk->vtime_snap += delta;

Here is how it works^Wfails

For simplicity tsk->vtime_snap starts at 0
HZ = 1000

CPU0 CPU1
sysexit()
account_system()
now == 0
delta = vtime_delta() <- 0ns
tsk->vtime_snap += delta; == 0ns

busy_loop(995us)

sysenter()
now == 996us
account_user()
delta = vtime_delta() <- 0ns
tsk->vtime_snap += delta == 0ns

sysexit()
account_system()
now == 1001us
delta = vtime_delta() <- 10000000ns

^^^^ Gets accounted to system

tsk->vtime_snap += delta; == 10000000ns

It's not different from the current jiffies based stuff at all. Same
failure mode.

Thanks,

tglx