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Date:   Wed, 9 Nov 2022 10:00:25 +0100
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Subject: Re: Deprecating and removing SLOB
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To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     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>,
        "linux-olinux-omap@vger.kernel.orgmap"@vger.kernel.org,
        openrisc@lists.librecores.org, linux-riscv@lists.infradead.org,
        linux-sh@vger.kernel.org
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 <CA+CK2bD-uVGJ0=9uc7Lt5zwY+2PM2RTcfOhxEd65S7TvTrJULA@mail.gmail.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <CA+CK2bD-uVGJ0=9uc7Lt5zwY+2PM2RTcfOhxEd65S7TvTrJULA@mail.gmail.com>
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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/arm/configs/clps711x_defconfig:CONFIG_SLOB=y
> arch/arm/configs/collie_defconfig:CONFIG_SLOB=y
> arch/arm/configs/multi_v4t_defconfig:CONFIG_SLOB=y
> arch/arm/configs/omap1_defconfig:CONFIG_SLOB=y
> arch/arm/configs/pxa_defconfig:CONFIG_SLOB=y
> arch/arm/configs/tct_hammer_defconfig:CONFIG_SLOB=y
> arch/arm/configs/xcep_defconfig:CONFIG_SLOB=y
> arch/openrisc/configs/or1ksim_defconfig:CONFIG_SLOB=y
> arch/openrisc/configs/simple_smp_defconfig:CONFIG_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
> arch/sh/configs/rsk7201_defconfig:CONFIG_SLOB=y
> arch/sh/configs/rsk7203_defconfig:CONFIG_SLOB=y
> arch/sh/configs/se7206_defconfig:CONFIG_SLOB=y
> arch/sh/configs/shmin_defconfig:CONFIG_SLOB=y
> arch/sh/configs/shx3_defconfig:CONFIG_SLOB=y
> kernel/configs/tiny.config:CONFIG_SLOB=y

Great point, thanks. Ccing. First mail here:
https://lore.kernel.org/all/CA%2BCK2bD-uVGJ0%3D9uc7Lt5zwY%2B2PM2RTcfOhxEd65S7TvTrJULA@mail.gmail.com/


>>
>> - Last time we discussed it [2], it seemed SLUB memory requirements can
>> be brought very close to SLOB's if needed. Of course it can never have
>> as small footprint as SLOB due to separate kmem_caches, but the
>> difference is not that significant, unless somebody still tries to use
>> Linux on very tiny systems (goes back to the previous point).
>>
>> Besides the smaller maintenance burden, removing SLOB would allow us to
>> do a useful API improvement - the ability to use kfree() for both
>> objects allocated by kmalloc() and kmem_cache_alloc(). Currently the
>> latter has to be freed by kmem_cache_free(), passing a kmem_cache
>> pointer in addition to the object pointer. With SLUB and SLAB, it is
>> however possible to use kfree() instead, as the kmalloc caches and the
>> rest of kmem_caches are the same and kfree() can lookup the kmem_cache
>> from object pointer easily for any of those. XFS has apparently did that
>> for years without anyone noticing it's broken on SLOB [3], and
>> legitimizing and expanding this would help some use cases beside XFS
>> (IIRC Matthew mentioned rcu-based freeing for example).
>>
>> However for SLOB to support kfree() on all allocations, it would need to
>> store object size of allocated objects (which it currently does only for
>> kmalloc() objects, prepending a size header to the object), but for
>> kmem_cache_alloc() allocations as well. This has been attempted in the
>> thread [3] but it bloats the memory usage, especially on architectures
>> with large ARCH_KMALLOC_MINALIGN, where the prepended header basically
>> has to occupy the whole ARCH_KMALLOC_MINALIGN block to be DMA safe.
>> There are ongoing efforts to reduce this minalign, but the memory
>> footprint would still increase, going against the purpose of SLOB, so
>> again it would be easier if we could just remove it.
>>
>> So with this thread I'm interested in hearing arguments/use cases for
>> keeping SLOB. There might be obviously users of SLOB whom this
>> conversation will not reach, so I assume the eventual next step would be
>> to deprecate it in a way that those users are notified when building a
>> new kernel and can raise their voice then. Is there a good proven way
>> how to do that for a config option like this one?
>>
>> Thanks,
>> Vlastimil
>>
>> [1] https://lpc.events/event/16/contributions/1272/ - slides in the
>> slabs.pdf linked there
>> [2]
>> https://lore.kernel.org/all/20211017135708.GA8442@kvm.asia-northeast3-a.c.our-ratio-313919.internal/#t
>> [3]
>> https://lore.kernel.org/all/20210930044202.GP2361455@dread.disaster.area/
>>
>>
>>