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

[Michal Simek]

Thomas Gleixner wrote:
> On Sat, 14 Nov 2009, Wu Zhangjin wrote:
>
> > From: Wu Zhangjin <wuzhangjin@xxxxxxxxx>
> >
> > 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.

please cc me on this discuss too. I have working ftrace implementation 
in my tree and I need improve timing too. I have similar patch as MIPS 
use. But I am not able to use it via timecounters. Something is weird 
there that's why I am open to find out any sensible and accepted solution.

Thanks,
Michal

> Thanks,
>
> 	tglx


--
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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