linux-kernel - Re: [PATCH 1/1] ftrace: fix typo in ftrace.txt file

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Date:	Wed, 20 May 2009 13:40:24 +0900
From:	GeunSik Lim <leemgs1@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	tglx <tglx@...utronix.de>, williams <williams@...hat.com>
Subject: Re: [PATCH 1/1] ftrace: fix typo in ftrace.txt file

2009/5/20 Steven Rostedt <rostedt@...dmis.org>:
>
> On Wed, 20 May 2009, GeunSik Lim wrote:
>
>>
>>
>> commit 43bf3fdddfd6b0ac65ef59bec3c79d9f9064e8bd
>> Author: GeunSik,Lim <leemgs1@...il.com>
>> Date:   Thu May 14 13:39:18 2009 +0900
>>
>>     ftrace: fix typo in ftrace.txt file.
>>
>>     Fix typo about chart to map the kernel priority to
>>     user land priorities.
>>
>>     About sched_setscheduler(2)
>>     Processes scheduled under SCHED_FIFO or SCHED_RR
>>     can have a static priority in the range 1 to 99.
>>
>>         Signed-off-by: GeunSik Lim <geunsik.lim@...sung.com>
>>
>> diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
>> index fd9a3e6..0e17632 100644
>> --- a/Documentation/trace/ftrace.txt
>> +++ b/Documentation/trace/ftrace.txt
>> @@ -518,9 +518,14 @@ priority with zero (0) being the highest priority and the nice
>>  values starting at 100 (nice -20). Below is a quick chart to map
>>  the kernel priority to user land priorities.
>>
>> -  Kernel priority: 0 to 99    ==> user RT priority 99 to 0
>> -  Kernel priority: 100 to 139 ==> user nice -20 to 19
>> -  Kernel priority: 140        ==> idle task priority
>> + Kernel priority: 0(high) to 99(low)    ==> user RT priority 99(high) to 1(low)
>
> Shouldn't that be:
>
>  Kernel priority: 0(high) to 98(low)    ==> user RT priority 99(high) to 1(low)
>
Thank you for your opinion.  I think about corrected expression for
quick chart to map between kernel space and user space in ftrace .

> 100 items does not map to 99. Which begs the question, what can have
> internal kernel priority 99?
>
ok.  Below example is about  kernel (static) priority 99 that you said.
please, refer to
http://www.kernel.org/doc/man-pages/online/pages/man2/sched_setscheduler.2.html
 webpage.

 For processes scheduled under one of the normal scheduling policies
(SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in
 scheduling decisions (it must be specified as 0).


int main (int argc, char **argv)
{	int pidnumber, ret;
	struct sched_param p;
	p.sched_priority = 0; <------ Kernel (static) priority.
	if (argc != 2) {
		printf("usage: setbatch <pid>\n");
		exit(-1);
	}
	pidnumber = atol(argv[1]);
         /* 3 = Number of SCHED_BATCH Constant */
	ret = sched_setscheduler(pidnumber, 3, &p);
	if (ret) {
		printf("could not set pid %d to SCHED_BATCH: err %d.\n", pid, ret);
		return -1;
	}
	printf("pid %d is SCHED_BATCH from now on.\n", pid);
	return 0;
}


>> + Kernel priority: 100(high) to 139(low) ==> user nice -20(high) to 19(low)
>> + Kernel priority: 140                   ==> idle task priority
>> +
>> +Processes scheduled with SCHED_OTHER or SCHED_BATCH must be assigned
>> +the static priority 0. Processes scheduled under SCHED_FIFO or SCHED_RR
>
> This line is confusing. "static priorty 0" really has no meaning here.
> Those now enter the "nice" priorities, and this statement does not
> represent what is shown in the ftrace output.
>
I agree with your opinion about exact understanding of ftrace output.
I will remove this additional description to avoid confusing
when we use this document as ftrace how to.

>> +can have a static priority in the range 1 to 99.
>> + (reference: $> man 2 sched_setscheduler)
>>
>
> -- Steve
>
>



-- 
Regards,
GeunSik Lim
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