linux-kernel - Re: [PATCH 4/4] perf/timer: 'perf timer' core code

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Date:	Thu, 17 Dec 2009 15:26:05 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] perf/timer: 'perf timer' core code

Hi Thomas,

Thomas Gleixner wrote:
> 
> Nothing to be sorry about. That's why we review code.
>  

Thanks.

>> Um, but not every timer has it's owner task, for example, if we start
>> a timer in interrupt handle function, this timer in not owns any tasks.
>> And itimer is started by userspace task so we can get it's owner, that
>> why i print hex address for timer/hrtimer, and print task name for itimer.
> 
> Well, lot's of timers have an owner task. At least all user space
> related ones. And if the timer is rearmed in interrupt context, then
> this does not change the owner at all.
> 

Sorry, i'm confused, for example, has below sequence:

Task1 running----->| (interrupt)
                   |------------- start timerT(start timerT in interrupt handler)

......
( After a while, schedule to another task, and interruption coming )
......

Task2 running----->| (interrupt)
                   |------------- start timerT again

Then, which task is the timerT owner?

Am I missed something?

>>>   How should that work ?
>>>
>> We put/get timer in a rb-tree base on the specify order, for example:
>> we default use this order:
>>
>> sort_dimension__add("timer", &default_cmp);
>> sort_dimension__add("itimer-type", &default_cmp);
>>
>> if timer_info->timer is bigger, we put it to left child, littler to right
>> child, if the timer_info->timer is the same, then we compare
>> timer_info->itimer_type.
> 
> Hmm, I wonder whether a hash table would be more efficient for the
> recording side.
> 

Um. i'll record it address your way.

> 
>> We search timer base on timer_info->timer and
>> timer_info->itimer_type(not timer_info->type), if we find the
>> timer's type is changed(for example, the timer is "ITIMER" before,
>> and change to "HRTIMER" later), is should a bug. this case is hardly
>> to happen but should catch it.
> 
> No, it's not a bug at all. You _can_ have a hrtimer and a timer_list
> timer at the same address in a trace. There are two ways to make that
> happen:
> 
>  1) kmalloc'ed memory contains a timer_list. timer operation is done,
>     memory is kfreed. Now another kmalloc gets the just freed memory
>     and has a hrtimer at the same address which was used by the
>     timer_list before.
> 
>  2) timer_list and hrtimer are also allocated on stack. There is no
>     guarantee that they are at different addresses.
>  

Yeah, my mistake.

> Simply because the macro hides the fact that this is an assignment of
> a value to a variable. That makes the code harder to read.
> 
>  	FILLL_RAW_FIELD_VALUE(event, value_sec, data);
> vs.	
> 	value_sec = get_value(data, event, "value_sec");
> 
> The latter is fast to parse and entirely clear.
> 

Yeah.

> Btw, you agreed above that the open coded call of raw_field_value()
> is clearer than the macro magic. :)
> 

Sorry, i misunderstand your mean before.

Thanks,
Xiao
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