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

[Thomas Gleixner <tglx@linutronix.de>]

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,

	tglx