MAXCPU preparations

[mdf at FreeBSD.org]

On Mon, Sep 27, 2010 at 9:21 AM, Sean Bruno <seanbru at yahoo-inc.com> wrote:
> On Mon, 2010-09-27 at 08:53 -0700, Julian Elischer wrote:
>> On 9/27/10 8:26 AM, Sean Bruno wrote:
>> > Does this look like an appropriate modification to libmemstat?
>> >
>> > Sean
>> >
>> >
>> > ==== //depot/yahoo/ybsd_7/src/lib/libmemstat/memstat.h#4
>> > - /home/seanbru/ybsd_7/src/lib/libmemstat/memstat.h ====
>> > @@ -28,12 +28,13 @@
>> >
>> >   #ifndef _MEMSTAT_H_
>> >   #define        _MEMSTAT_H_
>> > +#include<sys/param.h>
>> >
>> >   /*
>> >    * Number of CPU slots in library-internal data structures.  This
>> > should be
>> >    * at least the value of MAXCPU from param.h.
>> >    */
>> > -#define        MEMSTAT_MAXCPU  64
>> > +#define        MEMSTAT_MAXCPU  MAXCPU /* defined in
>> > sys/${ARCH}/include/param.h */
>> >
>>
>>
>> wouldn't it be better to do a sysctlbyname() and use the real value
>> for the system?
>>
>
> 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?

The maximum number of CPUs supported by a running instance will not
change.  Only, potentially, the current number of CPUs.  So a sysctl
to fetch the kernel's compiled-in MAXCPU is safe.

Thanks,
matthew