Re: [patch] sched: fix migration thread runtime bogosity

Peter Zijlstra
2012-08-03T20:39:53Z

On Fri, 2012-08-03 at 15:42 +0200, Mike Galbraith wrote:
> Greetings,
>
> I have two bug reports of absurd migration thread CPU usage, one of them
> with a link to a bisection..
>
> https://bugs.gentoo.org/show_bug.cgi?id=394487
>
> ..fingering d670ec13 - posix-cpu-timers: Cure SMP wobbles
>
> I reproduced with my -rt kernel and 3.4, but didn't manage to reproduce
> with the 3.0 NOPREEMPT kernel it was reported against.

Ah, I've seen similar reports, never managed to reproduce though.

> Signed-off-by: Mike Galbraith <efault@gmx.de>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 82ad284..82a78a6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -974,6 +974,13 @@ void sched_set_stop_task(int cpu, struct task_struct *stop)
> sched_setscheduler_nocheck(stop, SCHED_FIFO, &param);
>
> stop->sched_class = &stop_sched_class;
> +
> + /* Zero stale values for our non-accountable thread. */
> + stop->se.exec_start = 0;
> + stop->se.sum_exec_runtime = 0;
> + stop->se.prev_sum_exec_runtime = 0;
> + stop->stime = stop->stimescaled = 0;
> + stop->nvcsw = stop->nivcsw = 0;
> }
>
> cpu_rq(cpu)->stop = stop;

Now the question is, how did that stop thing get any time to begin with?
Are we hotplugging or somesuch sillyness?

Anyway, I think I like B best, could you re-submit as a proper patch so
I can press the magic button that queues stuff?