Zou Nan hai (on 08 Feb 2007 12:27:31 +0800) wrote: >On Thu, 2007-02-08 at 14:04, Keith Owens wrote: >> Zou Nan hai (on 08 Feb 2007 11:28:44 +0800) wrote: >> >Pin ar.kr2 of each CPU, so that smp_processor_id can use it. >> >> Historically ar.k2 has been reserved for debugging purposes, for >> example in ivt.S. Debuggers often need a location that can be used to >> track progress, it has to be somewhere that does not rely on TLB >> entries and is guaranteed to appear in MCA/INIT records - ar.k2 is >> perfect for this. >> > Ok, seems that current kr3 is only used by ia64_itc_printk_clock? >> Use Tony's suggestion of testing for a change in ar.k3 (guaranteed to >> be unique on every cpu) and caching the corresponding cpu number when >> it changes. >> > But why do we even need to cache it? > > It is already in a register if we put it to kr3. > so smp_processor_id() could be very fast. and later sys_getcpu can >also be very fast. ar.k3 is currently used for the address of the per-cpu data area, which speeds up access to all the per-cpu data. Changing ar.k3 to hold the cpu number means an extra array calculation and lookup for every per-cpu variable, slowing down the rest of the system. - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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