linux-kernel - RE: [PATCH v2 3/3] mm/slub: correct the default value of slub_min

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Date:   Tue, 5 Dec 2023 14:10:51 +0000
From:   "Song, Xiongwei" <Xiongwei.Song@...driver.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        "sxwjean@...com" <sxwjean@...com>
CC:     "vbabka@...e.cz" <vbabka@...e.cz>, "cl@...ux.com" <cl@...ux.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "penberg@...nel.org" <penberg@...nel.org>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
        "roman.gushchin@...ux.dev" <roman.gushchin@...ux.dev>,
        "corbet@....net" <corbet@....net>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 3/3] mm/slub: correct the default value of
 slub_min_objects in doc

Hi Hyeonggon,

> -----Original Message-----
> From: Hyeonggon Yoo <42.hyeyoo@...il.com>
> Sent: Tuesday, December 5, 2023 8:54 AM
> To: sxwjean@...com
> Cc: vbabka@...e.cz; cl@...ux.com; linux-mm@...ck.org; penberg@...nel.org;
> rientjes@...gle.com; iamjoonsoo.kim@....com; roman.gushchin@...ux.dev;
> corbet@....net; keescook@...omium.org; arnd@...db.de; akpm@...ux-foundation.org;
> gregkh@...uxfoundation.org; linux-doc@...r.kernel.org; linux-kernel@...r.kernel.org; Song,
> Xiongwei <Xiongwei.Song@...driver.com>
> Subject: Re: [PATCH v2 3/3] mm/slub: correct the default value of slub_min_objects in doc
> 
> On Sun, Dec 3, 2023 at 9:16 AM <sxwjean@...com> wrote:
> >
> > From: Xiongwei Song <xiongwei.song@...driver.com>
> >
> > There is no a value assigned to slub_min_objects by default, it always
> > is 0 that is initialized by compiler if no assigned value by command line.
> > min_objects is calculated based on processor numbers in calculate_order().
> > For more details, see commit 9b2cd506e5f2 ("slub: Calculate min_objects
> > based on number of processors.")
> >
> > Signed-off-by: Xiongwei Song <xiongwei.song@...driver.com>
> 
> While slub_min_objects equals zero by default, 'min_objects' overrides it to
> 4 * (fls(nr_cpus) + 1) when not set. so when slub_min_objects is not
> set, it would be
> equal to or higher than 4. I'm not sure this level of implementation
> detail is worth documenting.

commit 9b2cd506e5f2 ("slub: Calculate min_objects based on number of processors.")
has already given "processors min_objects" pair, do we really need to document
the specific detail?

> 
> Also, I think patch 2 should update Documentation/mm/slub.rst too.
> (slub_$param -> slab_param)
I think people can know slub_$params are still supported by
Documentation/mm/slub.rst, so we don't need to say the info again in
this file.  Is it better to do so just before removing slub_$params
completely?

Regards,
Xiongwei

> 
> > ---
> >  Documentation/mm/slub.rst | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/Documentation/mm/slub.rst b/Documentation/mm/slub.rst
> > index be75971532f5..1f4399581449 100644
> > --- a/Documentation/mm/slub.rst
> > +++ b/Documentation/mm/slub.rst
> > @@ -150,7 +150,7 @@ list_lock once in a while to deal with partial slabs. That overhead is
> >  governed by the order of the allocation for each slab. The allocations
> >  can be influenced by kernel parameters:
> >
> > -.. slub_min_objects=x          (default 4)
> > +.. slub_min_objects=x          (default 0)
> >  .. slub_min_order=x            (default 0)
> >  .. slub_max_order=x            (default 3 (PAGE_ALLOC_COSTLY_ORDER))
> >
> > --
> > 2.34.1
> >