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SubjectRe: [BUG nohz]: wrong user and system time accounting
On Tue, Apr 11, 2017 at 04:22:48PM +0200, Thomas Gleixner wrote:
> 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.

Yes you're right, I got confused again. So to fix this we could do our snapshots
at a frequency lower than HZ but still high enough to avoid overhead.

Something like TICK_NSEC / 2 ?

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