> -----Original Message-----
> From: linux-ia64-owner@xxxxxxxxxxxxxxx
> [mailto:linux-ia64-owner@xxxxxxxxxxxxxxx] On Behalf Of Luck, Tony
> Sent: 2007年2月8日 2:00
> To: Ken Chen; Yu, Fenghua
> Cc: linux-ia64@xxxxxxxxxxxxxxx
> Subject: RE: [PATCH] Hook up getcpu system call for IA64
>
> > Andi Kleen used to lecture me every so often why it is important to
> > have fast vgetcpu on x86-64 (I don't think ia64 is excluded from
> > that). His lecture is also all over the lkml mailing list.
>
> I dug though that thread ... it isn't ever explicitly stated, but
> it does appear that the intent is that a NUMA aware application
> would use a malloc() library that called getcpu() on every memory
> allocation. If that is the usage model, then I'll agree that
> getcpu() does need to be fast.
>
> One possible way to achieve that on ia64 would be to make use of the
> fact that the ar.k* registers are readable by applications, and
> specifically ar.k3 contains a per-cpu value (physical address of the
> kernel percpu area). *IF* (and that is a BIG IF) we were to guarantee
> to maintain ar.k3 as a per-cpu unique value, then we could implement
> a very fast (even faster than fsys.S) getcpu() as:
>
> #define __NR_getcpu 1304
>
> int
> getcpu(unsigned *cpup, unsigned *nodep, void *cachep)
> {
> static unsigned cpu = ~0, node = ~0;
> static unsigned long save_ar_k3;
> unsigned long ar_k3;
>
> asm volatile ("mov %0=ar.k3" : "=r" (ar_k3));
> if (cpu == ~0 || ar_k3 != save_ar_k3) {
> if (syscall(__NR_getcpu, &cpu, &node, 0) == -1)
> return -1;
> save_ar_k3 = ar_k3;
> }
> *cpup = cpu;
> *nodep = node;
>
> return 0;
> }
>
>
> Too ugly for words? Or worth serious consideration?
>
> -Tony
I think it is possible to pin ar.k3 to processor id, then change the raw_smp_processor_id() implementation.
This should be better than current raw_smp_processor_id() implementation.
Then we could implement sys_getcpu in fsys.S
That could not be very complex, only thing we need to care is exception handle in fsys code.
Thanks
Zou Nan hai
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