Re: [patch] fix SMT scheduler latency bug

[Ingo Molnar]

* Con Kolivas <kernel@xxxxxxxxxxx> 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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