Re: [PATCH 02/14] xen/sched: Constify name and opt_name in struct scheduler

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 06/04/2021 09:07, Jan Beulich wrote:
> On 05.04.2021 17:57, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > Both name and opt_name are pointing to literal string. So mark both of
> > the fields as const.
> >
> > Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> albeit ...
>
> > --- a/xen/common/sched/private.h
> > +++ b/xen/common/sched/private.h
> > @@ -272,8 +272,8 @@ static inline spinlock_t *pcpu_schedule_trylock(unsigned 
> > int cpu)
> >   }
> >   
> >   struct scheduler {
> > -    char *name;             /* full name for this scheduler      */
> > -    char *opt_name;         /* option name for this scheduler    */
> > +    const char *name;       /* full name for this scheduler      */
> > +    const char *opt_name;   /* option name for this scheduler    */
> ... I'd like to suggest considering at least the latter to become
> an array instead of a pointer - there's little point wasting 8
> bytes of storage for the pointer when the strings pointed to are
> all at most 9 bytes long (right now; I don't expect much longer
> ones to appear).
I have tried this simple/dumb change on top of my patch:

diff --git a/xen/common/sched/private.h b/xen/common/sched/private.h
index a870320146ef..ab2236874217 100644
--- a/xen/common/sched/private.h
+++ b/xen/common/sched/private.h
@@ -273,7 +273,7 @@ static inline spinlock_t 
*pcpu_schedule_trylock(unsigned int cpu)
 struct scheduler {
     const char *name;       /* full name for this scheduler      */
-    const char *opt_name;   /* option name for this scheduler    */
+    const char opt_name[9]; /* option name for this scheduler    */
     unsigned int sched_id;  /* ID for this scheduler             */
     void *sched_data;       /* global data pointer               */
     struct cpupool *cpupool;/* points to this scheduler's pool   */

GCC will throw an error:

core.c: In function ‘scheduler_init’:
core.c:2987:17: error: assignment of read-only variable ‘ops’
             ops = *schedulers[i];
                 ^
core.c:2997:21: error: assignment of read-only variable ‘ops’
                 ops = *schedulers[i];
                     ^

I don't particularly want to drop the const. So the code would probably 
need some rework.
My patch doesn't change the size of the structure, so I would prefer to 
keep this patch as-is.
Cheers,

--
Julien Grall