RFC: make UMTX hash table size configurable

[Konstantin Belousov]

On Wed, May 20, 2015 at 12:36:07PM -0400, Eric van Gyzen wrote:
> Adrian,
> 
> Good idea.  Looks good to me.
> 
> Eric
> 
> On 05/20/2015 12:28, Adrian Chadd wrote:
> > Hi,
> >
> > This patch makes the umtx hash table size configurable. It saves ~40k
> > of footprint from embedded images.
> >
> > (I can't seem to get phabricator to create a diff based on a set of
> > files, rather than the whole tree. Sigh.)
> >
> > adrian at lucy-11i386:~/work/freebsd/head-embedded/src % svn diff
> > sys/kern/ sys/conf/options
> > Index: sys/kern/kern_umtx.c
> > ===================================================================
> > --- sys/kern/kern_umtx.c    (revision 282934)
> > +++ sys/kern/kern_umtx.c    (working copy)
> > @@ -29,7 +29,7 @@
> >  __FBSDID("$FreeBSD$");
> >
> >  #include "opt_compat.h"
> > -#include "opt_umtx_profiling.h"
> > +#include "opt_umtx.h"
> >
> >  #include <sys/param.h>
> >  #include <sys/kernel.h>
> > @@ -184,7 +184,11 @@
> >               PRI_MAX_TIMESHARE : (td)->td_user_pri)
> >
> >  #define    GOLDEN_RATIO_PRIME    2654404609U
> > +#ifdef    UMTX_NUM_CHAINS
> > +#define    UMTX_CHAINS        UMTX_NUM_CHAINS
> > +#else
> >  #define    UMTX_CHAINS        512
> > +#endif    /* UMTX_NUM_CHAINS */
> >  #define    UMTX_SHIFTS        (__WORD_BIT - 9)
> >
> >  #define    GET_SHARE(flags)    \
> > Index: sys/conf/options
> > ===================================================================
> > --- sys/conf/options    (revision 282934)
> > +++ sys/conf/options    (working copy)
> > @@ -209,7 +209,8 @@
> >  SYSVSHM        opt_sysvipc.h
> >  SW_WATCHDOG    opt_watchdog.h
> >  TURNSTILE_PROFILING
> > -UMTX_PROFILING
> > +UMTX_PROFILING    opt_umtx.h
> > +UMTX_NUM_CHAINS    opt_umtx.h
> >  VFS_AIO
> >  VERBOSE_SYSINIT
> >  WLCACHE        opt_wavelan.h

It makes more sense to make the hash size a tunable.  The hash mask
would also need to be a variable.