linux-kernel - Re: [patch] f83f9ac causes tasks running at MAX

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Date:	Wed, 02 Dec 2009 13:49:10 +0100
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Williams <pwil3058@...pond.net.au>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch]  f83f9ac causes tasks running at MAX_PRIO

On Wed, 2009-12-02 at 12:46 +0100, Peter Zijlstra wrote:

> init_idle() doing:
>   idle->prio = idle->normal_prio = MAX_PRIO;
> 
> Which will propagate... like reported.
> 
> Now, since the idle-threads usually run on &idle_sched_class, nobody
> will actually look at their ->prio, so having that out-of-range might
> make sense.
> 
> Just needs to get fixed up when we fork a normal thread, which would be
> in sched_fork(), now your call to normal_prio() fixes this by setting
> everything to ->static_prio for SCHED_OTHER tasks, however
> 
> migration_call()
>   CPU_DEAD:
>     rq->idle->static_prio = MAX_PRIO;
> 
> spoils that too..

Darn.

> Ingo, any particular reason we set idle threads at MAX_PRIO? Can't we
> simply do something like below and be done with it?

Hysterical reasons?  That might have been a doorstop conversion kit for
O(1), but boots fine with CFS, and prio 40 tasks are history.

> ---
>  kernel/sched.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index c0e4e9d..5ad5a66 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -6963,7 +6963,6 @@ void __cpuinit init_idle(struct task_struct *idle,
> int cpu)
>  	__sched_fork(idle);
>  	idle->se.exec_start = sched_clock();
>  
> -	idle->prio = idle->normal_prio = MAX_PRIO;
>  	cpumask_copy(&idle->cpus_allowed, cpumask_of(cpu));
>  	__set_task_cpu(idle, cpu);
>  
> @@ -7667,7 +7666,6 @@ migration_call(struct notifier_block *nfb,
> unsigned long action, void *hcpu)
>  		spin_lock_irq(&rq->lock);
>  		update_rq_clock(rq);
>  		deactivate_task(rq, rq->idle, 0);
> -		rq->idle->static_prio = MAX_PRIO;
>  		__setscheduler(rq, rq->idle, SCHED_NORMAL, 0);
>  		rq->idle->sched_class = &idle_sched_class;
>  		migrate_dead_tasks(cpu);
> 

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