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

To: Wu Zhangjin <wuzhangjin@gmail.com>
Subject: Re: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak function
From: Steven Rostedt <rostedt@goodmis.org>
Date: Mon, 16 Nov 2009 10:01:52 -0500
Cc: 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>, Thomas Gleixner <tglx@linutronix.de>, 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
In-reply-to: <9dc81a7a9e5a292cccdf465c533a2b08d19d6021.1258177321.git.wuzhangjin@gmail.com>
Organization: Kihon Technologies Inc.
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <cover.1258177321.git.wuzhangjin@gmail.com> <9dc81a7a9e5a292cccdf465c533a2b08d19d6021.1258177321.git.wuzhangjin@gmail.com>
Reply-to: rostedt@goodmis.org
Sender: linux-mips-bounce@linux-mips.org
On Sat, 2009-11-14 at 14:33 +0800, 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.
> 
> The MIPS specific trace_clock_local() is coming in the next two patches.
> 
> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>

Acked-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve



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