Chen, Ken wrote on Thursday, February 23, 2006 6:34 PM > It's not efficient to use a per-cpu variable just to store > how many physical stack register a cpu has. Ever since the > incarnation of ia64 up till upcoming Montecito processor, that > variable has "glued" to 96. Having a variable in memory means > that the kernel is burning an extra cacheline access on every > syscall and kernel exit path. Such "static" value is better > served with the instruction patching utility exists today. > Convert ia64_phys_stacked_size_p8 into dynamic insn patching. > > This also has a pleasant side effect of eliminating access to > per-cpu area while psr.ic=0 in the kernel exit path. (fixable > for per-cpu DTC work, but why bother?) > > There are some concerns with the default value that the instruc- > tion encoded in the kernel image. It shouldn't be concerned. > The reasons are: > > (1) cpu_init() is called at CPU initialization. In there, we > find out physical stack register size from PAL and patch > two instructions in kernel exit code. The code in question > can not be executed before the patching is done. > > (2) current implementation stores zero in ia64_phys_stacked_size_p8, > and that's what the current kernel exit path loads the value with. > With the new code, it is equivalent that we store reg size 96 > in ia64_phys_stacked_size_p8, thus creating a better safety net. > Given (1) above can never fail, having (2) is just a bonus. > > > All in all, this patch allow one less memory reference in the kernel > exit path, thus reducing syscall and interrupt return latency; and > avoid polluting potential useful data in the CPU cache. I accidentally posted a variant of patch with IA64_NUM_PHYS_STACK_REG set to 0 for the purpose of testing out correctness of instruction patching. Change that value to 96 as advertised. - Ken --- linux-2.6.15/include/asm-ia64/processor.h.orig 2006-02-27 12:19:47.793181185 -0800 +++ linux-2.6.15/include/asm-ia64/processor.h 2006-02-27 12:19:56.312712330 -0800 @@ -20,7 +20,7 @@ #include <asm/ptrace.h> #include <asm/ustack.h> -#define IA64_NUM_PHYS_STACK_REG 0 +#define IA64_NUM_PHYS_STACK_REG 96 #define IA64_NUM_DBG_REGS 8 /* * Limits for PMC and PMD are set to less than maximum architected values - : 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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