linux-kernel - Re: [PATCH v8 01/16] tracing: convert trace_clock

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Date:	Mon, 16 Nov 2009 16:47:09 +0100
From:	Michal Simek <monstr@...str.eu>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Wu Zhangjin <wuzhangjin@...il.com>, rostedt@...dmis.org,
	Ralf Baechle <ralf@...ux-mips.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Nicholas Mc Guire <der.herr@...r.at>,
	David Daney <ddaney@...iumnetworks.com>,
	Richard Sandiford <rdsandiford@...glemail.com>,
	Patrik Kluba <kpajko79@...il.com>,
	"Maciej W . Rozycki" <macro@...ux-mips.org>,
	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	zhangfx@...ote.com, zhouqg@...il.com
Subject: Re: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak
 function

Thomas Gleixner wrote:
> On Sat, 14 Nov 2009, Wu Zhangjin wrote:
> 
>> From: Wu Zhangjin <wuzhangjin@...il.com>
>>
>> trace_clock_local() is based on the arch-specific sched_clock(), in X86,
>> it is tsc(64bit) based, which can give very high precision(about 1ns
>> with 1GHz). but in MIPS, the sched_clock() is jiffies based, which can
>> give only 10ms precison with 1000 HZ. which is not enough for tracing,
>> especially for Real Time system.
>>
>> so, we need to implement a MIPS specific sched_clock() to get higher
>> precision. There is a tsc like clock counter register in MIPS, whose
>> frequency is half of the processor, so, if the cpu frequency is 800MHz,
>> the time precision reaches 2.5ns, which is very good for tracing, even
>> for Real Time system.
>>
>> but 'Cause it is only 32bit long, which will rollover quickly, so, such
>> a sched_clock() will bring with extra load, which is not good for the
>> whole system. so, we only need to implement a arch-specific
>> trace_clock_local() for tracing. as a preparation, we convert it as a
>> weak function.
> 
> Hmm, I'm not convinced that this is really a huge overhead. 
> 
> First of all the rollover happens once every 10 seconds on a 800MHz
> machine. 
> 
> Secondly we have a lockless implementation of extending 32bit counters
> to 63 bit which is used at least by ARM to provide a high resolution
> sched_clock implementation. See include/linux/cnt32_63.h and the users
> in arch/
> 
> But that's a problem which can be discussed seperately and does not
> affect the rest of the tracing infrastructure. I really would
> recommend that you implement a sched_clock for the r4k machines based
> on cnt32_63 and measure the overhead. Having a fine granular
> sched_clock in general is probably not a bad thing.

please cc me on this discuss too. I have working ftrace implementation 
in my tree and I need improve timing too. I have similar patch as MIPS 
use. But I am not able to use it via timecounters. Something is weird 
there that's why I am open to find out any sensible and accepted solution.

Thanks,
Michal

> 
> Thanks,
> 
> 	tglx


-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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