Re: [patch] fix SMT scheduler latency bug

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



* 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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux