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

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Message-ID: <BANLkTim0SKAya-S_1h_zvS5rHW5ZOMG7Ow@mail.gmail.com>
Date:	Thu, 19 May 2011 09:41:27 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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 18, 2011 at 10:46 PM, Hillf Danton <dhillf@...il.com> wrote:
> On Wed, May 18, 2011 at 9:24 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> On Wed, 2011-05-18 at 21:19 +0800, Hillf Danton wrote:
>>
>>> Hi all
>>>
>>> The patch is prepared again, in which tests for both cpu and
>>> nr_cpus_allowed are dropped.
>>>
>>> The reason to drop nr_cpus_allowed is to make sure that the
>>> returned value is correct for both case that cpu == rq->cpu and
>>> case that cpu != rq->cpu.
>>
>> -ENOPARSE
>>
>> Why would we pick a task that can't migrate?
>>
> Hi Steven
>
> For migration, it is the case that cpu != rq->cpu, and
> if cpu is allowed by task's affinity, it is bug that task
> could not goto cpu because of nr_cpus_allowed since
> the nr_cpus_allowed is computed based on the cpus_allowed mask.

But for next_prio(), we just calculate the next highest migratible task
for a rq regardless on which cpu that task will run, say we let cpu=rq->cpu
to be second parameter of pick_next_highest_task_rt(), IOW, it has
the same effect as cpu = -1

Thanks,
Yong



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