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

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Message-ID: <9649f3fc-1802-6a0b-e492-d1ef6965d442@suse.cz>
Date:   Wed, 6 Dec 2023 17:33:38 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>, sxwjean@...com
Cc:     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,
        Xiongwei Song <xiongwei.song@...driver.com>
Subject: Re: [PATCH v2 3/3] mm/slub: correct the default value of
 slub_min_objects in doc

On 12/5/23 01:53, Hyeonggon Yoo wrote:
> 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.

We could say e.g. "(default: automaticaly scaled by number of cpus)"

> Also, I think patch 2 should update Documentation/mm/slub.rst too.
> (slub_$param -> slab_param)

I'd do it as separate patch, not part of patch 2.

>> ---
>>  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
>>