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

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Message-Id: <1255694372.7084.3.camel@falcon>
Date:	Fri, 16 Oct 2009 19:59:32 +0800
From:	Wu Zhangjin <wuzhangjin@...il.com>
To:	Américo Wang <xiyou.wangcong@...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, 2009-10-16 at 19:52 +0800, Américo Wang wrote:
> 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.
> 

Thanks, will use it in the next version.

Regards,
	Wu Zhangjin

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