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

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Date:	Mon, 16 Nov 2009 16:07:40 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Wu Zhangjin <wuzhangjin@...il.com>
cc:	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>,
	Michal Simek <monstr@...str.eu>,
	"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

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.

Thanks,

	tglx
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