Re: [PATCH v3 4/4] mm, slab_common: Make the loop for initializing KMALLOC_DMA start from 1

[Pengfei Li]

On Tue, Sep 10, 2019 at 6:26 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 9/10/19 3:26 AM, Pengfei Li wrote:
> > KMALLOC_DMA will be initialized only if KMALLOC_NORMAL with
> > the same index exists.
> >
> > And kmalloc_caches[KMALLOC_NORMAL][0] is always NULL.
> >
> > Therefore, the loop that initializes KMALLOC_DMA should start
> > at 1 instead of 0, which will reduce 1 meaningless attempt.
>
> IMHO the saving of one iteration isn't worth making the code more
> subtle. KMALLOC_SHIFT_LOW would be nice, but that would skip 1 + 2 which
> are special.
>

Yes, I agree with you.
This really makes the code more subtle.

> Since you're doing these cleanups, have you considered reordering
> kmalloc_info, size_index, kmalloc_index() etc so that sizes 96 and 192
> are ordered naturally between 64, 128 and 256? That should remove
> various special casing such as in create_kmalloc_caches(). I can't
> guarantee it will be possible without breaking e.g. constant folding
> optimizations etc., but seems to me it should be feasible. (There are
> definitely more places to change than those I listed.)
>

In the past two days, I am working on what you suggested.

So far, I have completed the coding work, but I need some time to make
sure there are no bugs and verify the impact on performance.

I will send v4 soon.

Thank you for your review and suggestions.

--
Pengfei

> > Signed-off-by: Pengfei Li <lpf.vector@xxxxxxxxx>
> > ---
> >   mm/slab_common.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index af45b5278fdc..c81fc7dc2946 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -1236,7 +1236,7 @@ void __init create_kmalloc_caches(slab_flags_t flags)
> >       slab_state = UP;
> >
> >   #ifdef CONFIG_ZONE_DMA
> > -     for (i = 0; i <= KMALLOC_SHIFT_HIGH; i++) {
> > +     for (i = 1; i <= KMALLOC_SHIFT_HIGH; i++) {
> >               struct kmem_cache *s = kmalloc_caches[KMALLOC_NORMAL][i];
> >
> >               if (s) {
> >
>