Re: [uClinux-dev] Kernel 2.6 size increase - get_current()?

[Bernardo Innocenti <bernie@develer.com>]

On Wednesday 23 July 2003 22:27, Christoph Hellwig wrote:

> On Wed, Jul 23, 2003 at 01:22:56PM -0700, David S. Miller wrote:
> > Drivers weren't audited much, and there's a lot of boneheaded
> > stuff in this area.  But these should be mostly identical
> > to what would happen on the 2.4.x side
>
> Please read the original message again - he stated that every single
> module in fs/ got alot bigger - if it gets smaller or at least the
> same size as 2.4 it's clearly a sign of inlines gone mad in the
> filesystem/VM code and we need to look at that.  If not we have to look
> elsewhere.

I have my humbling opinion:

In 2.4.20 (m68knommu):
-------------------------------------------------------------------------
#define current _current_task
-------------------------------------------------------------------------

In 2.6.0-test1 (m68knommu):
-------------------------------------------------------------------------
#define current get_current()
static inline struct task_struct *get_current(void)
{
        return(current_thread_info()->task);
}
static inline struct thread_info *current_thread_info(void)
{
        struct thread_info *ti;
        __asm__(
                "move.l %%sp, %0 \n\t"
                "and.l  %1, %0"
                : "=&d"(ti)
                : "d" (~(THREAD_SIZE-1))
                );
        return ti;
}
-------------------------------------------------------------------------

The latter expands to:

 0:	movel #-8192,%d0
 6:	movel %sp,%d2
 8:	andl %d0,%d2
 a:	moveal %d2,%a1
 c:	moveal %a1@,%a0
 e:	moveal %a0@(92),%a0
12:

It's a sequence of 6 instructions, 18 bytes long, clobbering 4 registers.
The compiler cannot see around it.

"current" is being used very lightly all over the kernel, like in this
code snippet from fs/open.c:

        old_fsuid = current->fsuid;
        old_fsgid = current->fsgid;
        old_cap = current->cap_effective;
        current->fsuid = current->uid;
        current->fsgid = current->gid;
        if (current->uid)
                cap_clear(current->cap_effective);
        else
                current->cap_effective = current->cap_permitted;

This takes 18*11 = 198 bytes just for invoking the 'current'
macro so many times.

Perhaps adding __attribute__((const)) on current_thread_info() and
get_current() would help eliminating some unnecessary accesses.

-- 
  // Bernardo Innocenti - Develer S.r.l., R&D dept.
\X/  http://www.develer.com/

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