linux-kernel - RE: [PATCH v2] mm/slub: Simplify get_partial

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Date: Wed, 13 Mar 2024 06:15:53 +0000
From: "Song, Xiongwei" <Xiongwei.Song@...driver.com>
To: Chengming Zhou <chengming.zhou@...ux.dev>,
        "sxwjean@...com"
	<sxwjean@...com>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg
	<penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim
	<iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
CC: "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] mm/slub: Simplify get_partial_node()

> On 2024/3/12 22:05, sxwjean@...com wrote:
> > From: Xiongwei Song <xiongwei.song@...driver.com>
> >
> > Remove the check of !kmem_cache_has_cpu_partial() because it is always
> > false, we've known this by calling kmem_cache_debug() before calling
> > remove_partial(), so we can remove the check.
> >
> > Meanwhile, redo filling cpu partial and add comment to improve the
> > readability.
> >
> > Signed-off-by: Xiongwei Song <xiongwei.song@...driver.com>
> > ---
> > Changes in v2:
> >  - Use "#if IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL)" to instead
> >    "if (IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL))" to fix build error.
> >    (Thanks Chengming Zhou)
> >  - Add __maybe_unused for partial_slabs to prevent compiler warning.
> >
> > v1:
> >  https://lore.kernel.org/linux-kernel/20240311132720.37741-1-sxwjean@me.com/T/
> > ---
> >  mm/slub.c | 24 +++++++++++++-----------
> >  1 file changed, 13 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index a3ab096c38c0..ab526960ee5b 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2588,7 +2588,7 @@ static struct slab *get_partial_node(struct kmem_cache *s,
> >  {
> >       struct slab *slab, *slab2, *partial = NULL;
> >       unsigned long flags;
> > -     unsigned int partial_slabs = 0;
> > +     unsigned int __maybe_unused partial_slabs = 0;
> >
> >       /*
> >        * Racy check. If we mistakenly see no partial slabs then we
> > @@ -2620,19 +2620,21 @@ static struct slab *get_partial_node(struct kmem_cache *s,
> >               if (!partial) {
> >                       partial = slab;
> >                       stat(s, ALLOC_FROM_PARTIAL);
> > -             } else {
> > -                     put_cpu_partial(s, slab, 0);
> > -                     stat(s, CPU_PARTIAL_NODE);
> > -                     partial_slabs++;
> > +
> > +                     /* Fill cpu partial if needed from next iteration, or break */
> > +                     if (IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL))
> > +                             continue;
> > +                     else
> > +                             break;
> >               }
> > -#ifdef CONFIG_SLUB_CPU_PARTIAL
> > -             if (!kmem_cache_has_cpu_partial(s)
> > -                     || partial_slabs > s->cpu_partial_slabs / 2)
> > +
> > +#if IS_ENABLED(CONFIG_SLUB_CPU_PARTIAL)
> 
> Hmm, these two CONFIG_SLUB_CPU_PARTIAL look verbose to me :(
> 
> How about using just one, maybe like this?
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2ef88bbf56a3..a018c715b648 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2620,19 +2620,16 @@ static struct slab *get_partial_node(struct kmem_cache *s,
>                 if (!partial) {
>                         partial = slab;
>                         stat(s, ALLOC_FROM_PARTIAL);
> +#ifdef CONFIG_SLUB_CPU_PARTIAL
>                 } else {

I don't like splitting  "if...else..." up with preprocessor directives personally. It's messy for me.