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Re: [PATCH 1/2] tracing: convert trace_clock_local() as weak function

To: Wu Zhangjin <wuzhangjin@gmail.com>
Subject: Re: [PATCH 1/2] tracing: convert trace_clock_local() as weak function
From: Américo Wang <xiyou.wangcong@gmail.com>
Date: Fri, 16 Oct 2009 19:52:14 +0800
Cc: linux-kernel@vger.kernel.org, linux-mips <linux-mips@linux-mips.org>, Ralf Baechle <ralf@linux-mips.org>, Thomas Gleixner <tglx@linutronix.de>, Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>, Manuel Lauss <manuel.lauss@googlemail.com>, Atsushi Nemoto <anemo@mba.ocn.ne.jp>
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In-reply-to: <4205779ae74d7c4144ee6cbf4e3f15f833646356.1255692619.git.wuzhangjin@gmail.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <4205779ae74d7c4144ee6cbf4e3f15f833646356.1255692619.git.wuzhangjin@gmail.com>
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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@gmail.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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