Re: [PATCH] posix-cpu-timers: clock_gettime(CLOCK_*_CPUTIME_ID) goes backward

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2008-12-26 at 09:43 +0100, Ingo Molnar wrote:
> * Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> wrote:
> 
> > @@ -321,7 +287,7 @@ static int cpu_clock_sample_group(const clockid_t which_clock,
> >  		cpu->cpu = cputime.utime;
> >  		break;
> >  	case CPUCLOCK_SCHED:
> > -		cpu->sched = cputime.sum_exec_runtime + task_delta_exec(p);
> > +		cpu->sched = cputime.sum_exec_runtime;
> >  		break;
> >  	}
> 
> hm, doesnt this regress precision?

No, he folds it into:

> +void thread_group_cputime(struct task_struct *p, struct task_cputime *times)
> +{
> +       unsigned long flags;
> +       struct rq *rq;
> +       u64 delta_exec = 0;
> +       struct task_cputime *tot;
> +       struct signal_struct *sig;
> +       int i;
> +
> +       sig = p->signal;
> +       if (unlikely(!sig) || !sig->cputime.totals) {
> +               times->utime = p->utime;
> +               times->stime = p->stime;
> +               times->sum_exec_runtime = task_total_exec(p);
> +               return;
> +       }
> +
> +       times->stime = times->utime = cputime_zero;
> +       times->sum_exec_runtime = 0;
> +
> +       rq = task_rq_lock(p, &flags);
> +
> +       if (task_current(rq, p)) {
> +               update_rq_clock(rq);
> +               delta_exec = rq->clock - p->se.exec_start;
> +               if ((s64)delta_exec < 0)
> +                       delta_exec = 0;
> +       }
> +
> +       for_each_possible_cpu(i) {
> +               tot = per_cpu_ptr(p->signal->cputime.totals, i);
> +               times->utime = cputime_add(times->utime, tot->utime);
> +               times->stime = cputime_add(times->stime, tot->stime);
> +               times->sum_exec_runtime += tot->sum_exec_runtime;
> +       }
> +       times->sum_exec_runtime += delta_exec;
> +
> +       task_rq_unlock(rq, &flags);
> +}

Which reminds me, why do we still have this crap in the kernel? I
thought we pretty much showed the per-cpu itimer thing was utter crap?
-- can we pretty please either revert that or apply
http://lkml.org/lkml/2008/11/24/183 ?

Also, I really don't like the above, we now do the per-cpu loop with the
RQ lock held...