Messages in this thread | | | Date | Mon, 16 Nov 2009 16:47:09 +0100 | From | Michal Simek <> | Subject | Re: [PATCH v8 01/16] tracing: convert trace_clock_local() as weak function |
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Thomas Gleixner wrote: > 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.
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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