Keith Owens (on Thu, 08 Feb 2007 17:37:54 +1100) wrote: >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. Correction: ar.k3 contains the physical address of the per-cpu data area, virtual access to per-cpu data goes via the cpu local TLB and does not rely on an ar.k<n> variable. ar.k3 is used in the MCA assembler handler, see GET_THIS_PADDR in include/asm-ia64/mca_asm.h and arch/ia64/kernel/mca_asm.S. - 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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