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Date: Mon, 3 Mar 2008 21:37:07 -1000 (HST)
From: Jeff Roberson <jroberson@chesapeake.net>
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To: David Xu <davidxu@FreeBSD.org>
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Cc: cvs-src@FreeBSD.org, Jeff Roberson <jeff@FreeBSD.org>,
	src-committers@FreeBSD.org, cvs-all@FreeBSD.org
Subject: Re: cvs commit: src/sys/kern init_sysent.c syscalls.c
 systrace_args.c src/sys/sys syscall.h syscall.mk sysproto.h
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On Tue, 4 Mar 2008, David Xu wrote:

> Jeff Roberson wrote:
>> 
>> On Tue, 4 Mar 2008, David Xu wrote:
>> 
>>> Jeff Roberson wrote:
>>> 
>>>>> One question is how I can determine the size of cpuset the kernel is
>>>>> using ?
>>>> 
>>>> I wrote it to tolerate user masks that were much larger than the kernel 
>>>> mask.  I set the default CPU_SETSIZE in userspace to 128 and in kernel 
>>>> it's MAXCPU.  So in practice an application shouldn't have to redefine 
>>>> CPU_SETSIZE.  If your set is too small the kernel will return ERANGE 
>>>> however.  Unfortunately, if your set is larger than the kernel's 
>>>> CPU_MAXSIZE it'll also return ERANGE.  Maybe I should use different 
>>>> errnos for those cases.
>>>> 
>>> 
>>> From my point, userland has to write some urgly code to guess what
>>> kernel code wants, it is rather frustrate.
>> 
>> You can use sysctl kern.smp.maxcpus to get the precise size.
>> 
>
> if kern.smp.maxcpus is a stable ABI, I may use it, can it be guaranteed?
> I saw following code in kern_cpuset.c, obviously, maxcpus is not
> respected.
>
> if (uap->cpusetsize < CPU_SETSIZE || uap->cpusetsize > CPU_MAXSIZE)
>    return (ERANGE);
>
>

CPU_SETSIZE in the kernel is defined to MAXCPU.  We can consider 
kern.smp.maxcpus a stable abi.  I can make a note next to it and put it in 
the cpuset man page.