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SubjectRe: Regression in gdm-2.18 since 2.6.24
On Sat, Apr 05, 2008 at 08:10:43PM +0530, Srivatsa Vaddagiri wrote:
>
> Given that you seem to be seeing the problem even without
> CONFIG_GROUP_SCHED, only the second hunk of the patch seems to be making
> a difference for your problem i.e just the hunk below applied on
> 2.6.25-rc8 (to kernel/sched_fair.c) should fix your problem too:
>
> @@ -1145,7 +1145,7 @@ static void check_preempt_wakeup(struct
> * More easily preempt - nice tasks, while not making
> * it harder for + nice tasks.
> */
> - if (unlikely(se->load.weight > NICE_0_LOAD))
> + if (unlikely(se->load.weight != NICE_0_LOAD))
> gran = calc_delta_fair(gran, &se->load);
>
> if (pse->vruntime + gran < se->vruntime)
>
> [The first hunk is a no-op under !CONFIG_GROUP_SCHED, since
> entity_is_task() is always 1 for !CONFIG_GROUP_SCHED]
>
> This second hunk changes how fast + or - niced tasks get preempted.
>
> 2.6.25-rc8 (Bad case):
> Sets preempt granularity for + niced tasks at 5ms (1 CPU)
>
> 2.6.25-rc8 + the hunk above (Good case):
> Sets preempt granularity for + niced tasks at >5ms
>
Well, I'm no longer sure exactly what was in the config, but after
I had confirmed the reversion would fix 2.6.24.4 I _did_ try just
the second part of the patch applied to 2.6.25-rc8 and it gave a 60%
success rate across 10 tests.
>
> So bumping up preempt granularity for + niced tasks seems to make things
> work for you. IMO the deeper problem lies somewhere else (perhaps is
> some race issue in gdm itself), which is easily exposed with 2.6.25-rc8
> which lets + niced tasks be preempted quickly.
>

I agree this is probably exposing a problem somewhere else.

> To help validate this, can you let us know the result of tuning preempt
> granularity on native 2.6.25-rc8 (without any patches applied and
> CONFIG_GROUP_SCHED disabled)?
>
> # echo 100000000 > /proc/sys/kernel/sched_wakeup_granularity_ns
>
> To check if echo command worked, do:
>
> # cat /proc/sys/kernel/sched_wakeup_granularity_ns
>
> It should return 100000000.
>
> Now try shutting down thr' gdm and pls let me know if it makes a
> difference.
>
> --
> Regards,
> vatsa

Will do, but it might be a day or so before I can get to this.

Thanks.

Ken
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