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

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Date:	Thu, 12 May 2011 20:06:06 +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 Wed, May 11, 2011 at 09:44:04PM +0800, Hillf Danton wrote:
> On Wed, May 11, 2011 at 4:43 PM, Yong Zhang <yong.zhang0@...il.com> wrote:
> > 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'
> >
> If cpu is not cared, how to determine whether it is allowed for task to run?

pick_next_highest_task_rt() can be used to get the next highest pullable
task on a certain rq(regradless on which cpu that task could run). but
currently we have no such kind of caller.

> 
> >> 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.
> >
> If cpu is allowed for task to run, then why more cpus are enforced?

I think you can take a look at next_prio(), it just calculate the
next highest task on the current cpu; in this case,
cpumask_test_cpu(cpu, &p->cpus_allowed) will be true for the most
of time, but maybe that task is bound to this cpu.

Thanks,
Yong
> 
> thanks
>           Hillf
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