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Message-ID: <e4076fcb-9736-937d-634b-5b8fb342723e@opensource.wdc.com>
Date:   Sat, 12 Nov 2022 10:40:53 +0900
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Subject: Re: Deprecating and removing SLOB
Content-Language: en-US
To:     Conor Dooley <conor@kernel.org>, Vlastimil Babka <vbabka@suse.cz>
Cc:     Pasha Tatashin <pasha.tatashin@soleen.com>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Pekka Enberg <penberg@kernel.org>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Rustam Kovhaev <rkovhaev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Russell King <linux@armlinux.org.uk>,
        Alexander Shiyan <shc_work@mail.ru>,
        Aaro Koskinen <aaro.koskinen@iki.fi>,
        Janusz Krzysztofik <jmkrzyszt@gmail.com>,
        Tony Lindgren <tony@atomide.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Rich Felker <dalias@libc.org>, Jonas Bonn <jonas@southpole.se>,
        Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
        Stafford Horne <shorne@gmail.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        openrisc@lists.librecores.org, linux-riscv@lists.infradead.org,
        linux-sh@vger.kernel.org,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Conor.Dooley@microchip.com, Paul Cercueil <paul@crapouillou.net>
References: <b35c3f82-f67b-2103-7d82-7a7ba7521439@suse.cz>
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 <c1caa5ce-eeaf-8038-2dea-051c98aade45@suse.cz> <Y260tkNHc2vFITJ3@spud>
From:   Damien Le Moal <damien.lemoal@opensource.wdc.com>
Organization: Western Digital Research
In-Reply-To: <Y260tkNHc2vFITJ3@spud>
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On 11/12/22 05:46, Conor Dooley wrote:
> On Fri, Nov 11, 2022 at 11:33:30AM +0100, Vlastimil Babka wrote:
>> On 11/8/22 22:44, Pasha Tatashin wrote:
>>> On Tue, Nov 8, 2022 at 10:55 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>>>
>>>> Hi,
>>>>
>>>> as we all know, we currently have three slab allocators. As we discussed
>>>> at LPC [1], it is my hope that one of these allocators has a future, and
>>>> two of them do not.
>>>>
>>>> The unsurprising reasons include code maintenance burden, other features
>>>> compatible with only a subset of allocators (or more effort spent on the
>>>> features), blocking API improvements (more on that below), and my
>>>> inability to pronounce SLAB and SLUB in a properly distinguishable way,
>>>> without resorting to spelling out the letters.
>>>>
>>>> I think (but may be proven wrong) that SLOB is the easier target of the
>>>> two to be removed, so I'd like to focus on it first.
>>>>
>>>> I believe SLOB can be removed because:
>>>>
>>>> - AFAIK nobody really uses it? It strives for minimal memory footprint
>>>> by putting all objects together, which has its CPU performance costs
>>>> (locking, lack of percpu caching, searching for free space...). I'm not
>>>> aware of any "tiny linux" deployment that opts for this. For example,
>>>> OpenWRT seems to use SLUB and the devices these days have e.g. 128MB
>>>> RAM, not up to 16 MB anymore. I've heard anecdotes that the performance
>>>> SLOB impact is too much for those who tried. Googling for
>>>> "CONFIG_SLOB=y" yielded nothing useful.
>>>
>>> I am all for removing SLOB.
>>>
>>> There are some devices with configs where SLOB is enabled by default.
>>> Perhaps, the owners/maintainers of those devices/configs should be
>>> included into this thread:
>>>
>>> tatashin@soleen:~/x/linux$ git grep SLOB=y
> 
>>> arch/riscv/configs/nommu_k210_defconfig:CONFIG_SLOB=y
>>> arch/riscv/configs/nommu_k210_sdcard_defconfig:CONFIG_SLOB=y
>>> arch/riscv/configs/nommu_virt_defconfig:CONFIG_SLOB=y
> 
>>
>> Turns out that since SLOB depends on EXPERT, many of those lack it so
>> running make defconfig ends up with SLUB anyway, unless I miss something.
>> Only a subset has both SLOB and EXPERT:
>>
>>> git grep CONFIG_EXPERT `git grep -l "CONFIG_SLOB=y"`
> 
>> arch/riscv/configs/nommu_virt_defconfig:CONFIG_EXPERT=y
> 
> I suppose there's not really a concern with the virt defconfig, but I
> did check the output of `make nommu_k210_defconfig" and despite not
> having expert it seems to end up CONFIG_SLOB=y in the generated .config.
> 
> I do have a board with a k210 so I checked with s/SLOB/SLUB and it still
> boots etc, but I have no workloads or w/e to run on it.

I will try with SLUB over the weekend.

-- 
Damien Le Moal
Western Digital Research