Re: MAXCPU preparations

On Monday, September 27, 2010 5:21:31 pm Robert Watson wrote:
> 
> On Mon, 27 Sep 2010, Sean Bruno wrote:
> 
> >> wouldn't it be better to do a sysctlbyname() and use the real value for the 
> >> system?
> 
> libmemstat contains some useful sample code showing how this might be done.
> 
> > That was my initial thought (as prodded by scottl and peter).
> >
> > If it is made dynamic, could this be opening a race condition where the call 
> > to sysctlbyname() returns a count of CPUS that is in turn changed by the 
> > offlining of a CPU?  Or am I thinking to much about this?
> 
> Yes, you are.  MAXCPU is a compile-time constant for kernel builds, so (at 
> least a the world is today), that can't happen.
> 
> I think there's a reasonable argument that MEMSTAT_MAXCPU should be phased out 
> and all internal structures in libmemstat should be dynamically sized. 
> However, core counts aren't growing that fast, and it's quite a bit of work, 
> and probably not worth it just yet.
> 
> I'm somewhat averse to using MAXCPU in libmemstat, however, because MAXCPU is 
> actually not a constant in the general case: FreeBSD/i386, for example, 
> regularly uses two different values: 1 for !SMP kernels, and 32 for SMP 
> kernels.  That's why libmemstat encodes its own value, for better or worse.
> 
> A reasonable alternative would be to replace 32 with MAXCPU * 2, or if we're 
> feeling particularly optimistic, MAXCPU * 4.  Or just another big number, like 
> 256.

A prerequisite for this idea though is that MAXCPU needs to be fixed to
export the SMP value to userland rather than the UP value.  For example, this
code in amd64/include/param.h:

#if defined(SMP) || defined(KLD_MODULE)
#define MAXCPU		32
#else
#define MAXCPU		1
#endif

would need to change so that !_KERNEL is in the conditional.  I do think we
should avoid using MAXCPU in userland as much as possible and use the
existing sysctl for mp_maxid instead to dynamically allocate arrays.

Also, I think we should either fix MAXCPU to export the SMP value to
userland, or hide it from userland completely.  Exporting the UP value is
Just Wrong (tm).

-- 
John Baldwin