Messages in this thread | | | From | Con Kolivas <> | Subject | Re: [patch] fix SMT scheduler latency bug | Date | Thu, 23 Jun 2005 10:03:28 +1000 |
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On Thu, 23 Jun 2005 09:32, Ingo Molnar wrote: > * Con Kolivas <kernel@kolivas.org> 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.
Agreed.
> 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?
In dependent_sleeper() we return 1 only to prevent p from scheduling. This second condition does not return 1 from dependent_sleeper() so p will still go ahead and schedule. This second condition only affects the scheduling on the smt sibling.
About the only scenario I can envision a high priority task being delayed with the code as it currently is in 2.6.12-mm1 is with a high priority task being on the expired array and a low priority task being delayed on the active array. This still should not create large latencies unless array swapping is significantly delayed. I considered adding a check for this originally but it seemed to be unnecessary extra complexity since an expired task by design is expected to be delayed more anyway.
Cheers, Con
Cheers, Con [unhandled content-type:application/pgp-signature] | |