linux-kernel - Re: [PATCH] sched: correct how RT task is picked

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Date:	Wed, 11 May 2011 16:43:49 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] sched: correct how RT task is picked

On Tue, May 10, 2011 at 9:04 PM, Hillf Danton <dhillf@...il.com> wrote:
> When picking RT task for given CPU,
> [1] if the cpu is invalid for cpumask test, right result could not be

'cpu is invalid' means weather we care it or not, it's not real 'invalid'

> reached even by further checking nr_cpus_allowed,
> on the other hand, the input cpu is valid in two cases that
> pick_next_highest_task_rt() is called, thus the invalid input cpu
> looks over-concern.
> [2] if the cpu is valid for cpumask test, further checking
> nr_cpus_allowed looks overwork, since it is computed based on
> cpus_allowed,

No, cpumask_test_cpu(cpu, &p->cpus_allowed) doesn't mean
p->rt.nr_cpus_allowed > 1.

Thanks,
Yong

> what is more, the combination of cpus_allowed and nr_cpus_allowed
> could lead to incorrect result if the input cpu == rq->cpu, as in the
> case of next_prio() where no pulling task is concerned.
>
> In this work, invalid cpu is not removed but leads to negative result,
> but nr_cpus_allowed is.
>
> Signed-off-by: Hillf Danton <dhillf@...il.com>
> ---
>
> --- a/kernel/sched_rt.c 2011-04-27 11:48:50.000000000 +0800
> +++ b/kernel/sched_rt.c 2011-05-10 20:30:38.000000000 +0800
> @@ -1149,10 +1149,12 @@ static void deactivate_task(struct rq *r
>
>  static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
>  {
> -       if (!task_running(rq, p) &&
> -           (cpu < 0 || cpumask_test_cpu(cpu, &p->cpus_allowed)) &&
> -           (p->rt.nr_cpus_allowed > 1))
> -               return 1;
> +       if (!task_running(rq, p)) {
> +               if (cpu < 0)
> +                       return 0;
> +               if (cpumask_test_cpu(cpu, &p->cpus_allowed))
> +                       return 1;
> +       }
>        return 0;
>  }
>



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