linux-kernel - Re: [RFC] oom-kill: give the dying task a higher priority

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Date:	Fri, 28 May 2010 16:52:23 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	balbir@...ux.vnet.ibm.com,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Mel Gorman <mel@....ul.ie>, williams@...hat.com
Subject: Re: [RFC] oom-kill: give the dying task a higher priority

On Fri, May 28, 2010 at 2:59 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> RT Task
>>
>> void non-RT-function()
>> {
>>    system call();
>>    buffer = malloc();
>>    memset(buffer);
>> }
>> /*
>>  * We make sure this function must be executed in some millisecond
>>  */
>> void RT-function()
>> {
>>    some calculation(); <- This doesn't have no dynamic characteristic
>> }
>> int main()
>> {
>>    non-RT-function();
>>    /* This function make sure RT-function cannot preempt by others */
>>    set_RT_max_high_priority();
>>    RT-function A();
>>    set_normal_priority();
>>    non-RT-function();
>> }
>>
>> We don't want realtime in whole function of the task. What we want is
>> just RT-function A.
>> Of course, current Linux cannot make perfectly sure RT-functionA can
>> not preempt by others.
>> That's because some interrupt or exception happen. But RT-function A
>> doesn't related to any dynamic characteristic. What can justify to
>> preempt RT-function A by other processes?
>
> As far as my observation, RT-function always have some syscall. because pure
> calculation doesn't need deterministic guarantee. But _if_ you are really
> using such priority design. I'm ok maximum NonRT priority instead maximum
> RT priority too.

Hmm. It's just example. but it would be not good exmaple.
Let's change it with this.

void RT-function()
{
     int result = some calculation(); <- This doesn't have no dynamic
characteristic
     *mmap_base = result; <-- mmap_base is mapped by GPIO device.
}

Could we allow preemption of this RT function due to other task's
memory pressure?
Of course, Linux is not Hard RT featured OS, I think. So I thinks it
is a policy problem.
If we think system memory pressure is more important than RT task and
we _all_ agree such policy, we can allow it.

But I don't hope it.

-- 
Kind regards,
Minchan Kim
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