linux-kernel - Re: How can I migrate a currently running task?

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Message-ID: <20080627184815.GB10511@sgi.com>
Date:	Fri, 27 Jun 2008 13:48:16 -0500
From:	Robin Holt <holt@....com>
To:	xialiang <xiaiaxaxi@...u.edu.cn>
Cc:	Bart Van Assche <bart.vanassche@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: How can I migrate a currently running task?

On Fri, Jun 27, 2008 at 09:38:05PM +0800, xialiang wrote:
> Quoting Bart Van Assche <bart.vanassche@...il.com>:
>
>> On Fri, Jun 27, 2008 at 3:17 PM, 夏亮 <xiaiaxaxi@...u.edu.cn> wrote:
>>> I am working on Intel Duo Core with Linux OS 2.6.21, and I'd like to
>>> migrate task from one cpu to another cpu.
>>> In SMP systems, load_balance() function uses move_tasks() to move processes
>>> from source runqueue to local runqueue, but it does not move a currently
>>> running task. If I want to migrate a currently running task from source
>>> runqueue to local runqueue, how can I do? Any suggestion is preferred.
>>
>> Are you familiar with the glibc pthread_setaffinity_np() function
>> and/or the sched_setaffinity() system call ?
>>
>> Bart.
>>
>
> Yes. I know sched_setaffinity(), it sets cpu_mask of a task. I want to  
> use it in a timer interrupt( scheduler_tick() ), can I?

Don't use it from a timer handler.  You can use migrate_task() to move the
task, but your email from a couple days ago said you wanted to move the
task at the head of the runqueue due to cpu heat or something like that.
That is a very imprecise way to move the task as any unfortunate task
that happens to be running when the timer tick occurs could get migrated,
not the one generating the work.

I think you really want to look at the other areas of the kernel which do
stuff like throttling the cpu.  Just moving a task off the cpu does not
prevent it from being used for another compute intesive load.  You could
take the cpu offline.  I guess to do that, I would use schedule_work()
or kthread_create() to get out of the timer context and into a regular
thread context then take the cpu offline from there.

Good Luck,
Robin
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