linux-kernel - Re: [PATCH 1/2] tracing: convert trace_clock

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Date:	Fri, 16 Oct 2009 19:52:14 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Wu Zhangjin <wuzhangjin@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-mips <linux-mips@...ux-mips.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Manuel Lauss <manuel.lauss@...glemail.com>,
	Atsushi Nemoto <anemo@....ocn.ne.jp>
Subject: Re: [PATCH 1/2] tracing: convert trace_clock_local() as weak
	function

On Fri, Oct 16, 2009 at 07:38:24PM +0800, Wu Zhangjin wrote:
>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 patch.
>
>Signed-off-by: Wu Zhangjin <wuzhangjin@...il.com>
>---
> kernel/trace/trace_clock.c |    2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
>diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
>index 20c5f92..a04dc18 100644
>--- a/kernel/trace/trace_clock.c
>+++ b/kernel/trace/trace_clock.c
>@@ -26,7 +26,7 @@
>  * Useful for tracing that does not cross to other CPUs nor
>  * does it go through idle events.
>  */
>-u64 notrace trace_clock_local(void)
>+u64 __attribute__((weak)) notrace trace_clock_local(void)

We have __weak.


> {
> 	unsigned long flags;
> 	u64 clock;
>-- 
>1.6.2.1
>
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