svn commit: r324863 - in head/sys: kern sys

[Mateusz Guzik]

On Sun, Oct 22, 2017 at 04:24:41PM +0200, Hans Petter Selasky wrote:
> On 10/22/17 15:42, Mateusz Guzik wrote:
> > Author: mjg
> > Date: Sun Oct 22 13:42:56 2017
> > New Revision: 324863
> > URL: https://svnweb.freebsd.org/changeset/base/324863
> >
> > Log:
> >    Change kdb_active type to u_char.
> >    Fixes warnings from gcc and keeps the small size. Perhaps nesting
should be moved
> >    to another variablle.
> >    Reported by:    ngie
> >
> > Modified:
> >    head/sys/kern/subr_kdb.c
> >    head/sys/sys/kdb.h
> >
> > Modified: head/sys/kern/subr_kdb.c
> >
==============================================================================
> > --- head/sys/kern/subr_kdb.c    Sun Oct 22 12:12:52 2017    (r324862)
> > +++ head/sys/kern/subr_kdb.c    Sun Oct 22 13:42:56 2017    (r324863)
> > @@ -50,7 +50,7 @@ __FBSDID("$FreeBSD$");
> >   #include <machine/smp.h>
> >   #endif
> > -bool __read_frequently kdb_active = 0;
> > +u_char __read_frequently kdb_active = 0;
> >   static void *kdb_jmpbufp = NULL;
> >   struct kdb_dbbe *kdb_dbbe = NULL;
> >   static struct pcb kdb_pcb;
> >
> > Modified: head/sys/sys/kdb.h
> >
==============================================================================
> > --- head/sys/sys/kdb.h    Sun Oct 22 12:12:52 2017    (r324862)
> > +++ head/sys/sys/kdb.h    Sun Oct 22 13:42:56 2017    (r324863)
> > @@ -59,7 +59,7 @@ struct kdb_dbbe {
> >       };                        \
> >       DATA_SET(kdb_dbbe_set, name##_dbbe)
> > -extern bool kdb_active;            /* Non-zero while in debugger. */
> > +extern u_char kdb_active;        /* Non-zero while in debugger. */
> >   extern int debugger_on_panic;        /* enter the debugger on panic.
*/
> >   extern struct kdb_dbbe *kdb_dbbe;    /* Default debugger backend or
NULL. */
> >   extern struct trapframe *kdb_frame;    /* Frame to kdb_trap(). */
> >
> >
>
> Should we add __aligned(8) to this definition?
>
> ./systm.h:#define    __read_frequently
__section(".data.read_frequently")
>
> It will prevent commonly read variables from residing in two different
> cache-lines on x86 and amd64 at least???
>

I don't follow. This would *increase* alignemnt requirement and in
particular prevent bool variables from being put in consecutive bytes.

To answer the question from your other e-mail, the bigger the type the
worse it is as it takes more space. The idea is to change all frequently
read and effectively bool variables from int to bool so that more of
them fit in one cacheline.

Right now there is nothing to nicely sort them to get rid of holes, but
I'm tinkering with automagic size addition to section name.

-- 
Mateusz Guzik <mjguzik gmail.com>