* Con Kolivas <[email protected]> wrote:
> > task_timeslice(p) is indeed constant over time, but smt_curr->time_slice
> > is not. So this condition opens up the possibility of a lower prio
> > thread accumulating a larger ->time_slice and thus reversing the
> > priority equation.
>
> I'm not clear on how the value of ->time_slice can ever grow to larger
> than task_timeslice(p). It starts at task_timeslice(p) and decrements
> till it gets to 0 when it refills again.
I was thinking abut sched_exit(), there we let unused child timeslices
'flow back' into the parent thread, if the child thread was shortlived.
The check there does:
if (p->first_time_slice) {
p->parent->time_slice += p->time_slice;
if (unlikely(p->parent->time_slice > task_timeslice(p)))
p->parent->time_slice = task_timeslice(p);
}
notice that we check parent->time_slice against the child's
task_timeslice(p), not against task_timeslice(p->parent). So if the
child thread got reniced, it could cause a higher-than-normal amount of
timeslices. But this should be a rare scenario, and the above code is
more of a bug than a feature (will send a patch for it tomorrow), and it
should not affect the workloads i was testing.
lets take a look at the second condition again:
if ((p->time_slice * (100 - sd->per_cpu_gain) / 100) >
task_timeslice(smt_curr))
resched_task(smt_curr);
if this condition is true then we trigger a preemption at smt_curr. Now
in the bug scenario, 'p' is a highprio task and smt_curr is a lowprio
task. If p->time_slice (which fluctuates between task_timeslice(p) and
0) happens to be low enough, preemption wont be triggered and we lose a
wakeup in essence - 'p', despite being the highest-prio task around,
wont be run until some CPU runs schedule() voluntarily. Ok?
Ingo
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