RE: [PATCH] Hook up getcpu system call for IA64

From: Zou, Nanhai <nanhai.zou_at_intel.com>
Date: 2007-02-08 12:25:36
> -----Original Message-----
> From: linux-ia64-owner@vger.kernel.org
> [mailto:linux-ia64-owner@vger.kernel.org] On Behalf Of Luck, Tony
> Sent: 200728 2:00
> To: Ken Chen; Yu, Fenghua
> Cc: linux-ia64@vger.kernel.org
> 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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Received on Thu Feb 08 12:26:32 2007

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