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Re: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak functi

To: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak function
From: Wu Zhangjin <wuzhangjin@gmail.com>
Date: Mon, 16 Nov 2009 23:21:08 +0800
Cc: rostedt@goodmis.org, Ralf Baechle <ralf@linux-mips.org>, Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>, Nicholas Mc Guire <der.herr@hofr.at>, David Daney <ddaney@caviumnetworks.com>, Richard Sandiford <rdsandiford@googlemail.com>, Patrik Kluba <kpajko79@gmail.com>, Michal Simek <monstr@monstr.eu>, "Maciej W . Rozycki" <macro@linux-mips.org>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, zhangfx@lemote.com, zhouqg@gmail.com
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On Mon, 2009-11-16 at 16:07 +0100, Thomas Gleixner wrote:
> On Sat, 14 Nov 2009, Wu Zhangjin wrote:
> 
> > From: Wu Zhangjin <wuzhangjin@gmail.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 very much for your prompt, 

Will have a look at cnt32_to_63() in include/linux/cnt32_to_63.h later.

Thanks & Regards,
        Wu Zhangjin


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